LitOptions class to hold backend options

[GSterbrant]

Description

Fixes #4871.

The backend options for the lit shader are now defined in their own class, which makes it possible to know what options are available.

Additionally, this allows us to validate options which might have moved to the backend object, and inform developers if they are still expecting the option to be there. This primarily pertains to options set through the onUpdateShader callback.

[mvaligursky]

The docs for these should probably move to appropriate classes too

engine/src/scene/materials/standard-material.js

Lines 495 to 564 in 636571e

	* @property {UpdateShaderCallback} onUpdateShader A custom function that will be called after all
	* shader generator properties are collected and before shader code is generated. This function
	* will receive an object with shader generator settings (based on current material and scene
	* properties), that you can change and then return. Returned value will be used instead. This is
	* mostly useful when rendering the same set of objects, but with different shader variations based
	* on the same material. For example, you may wish to render a depth or normal pass using textures
	* assigned to the material, a reflection pass with simpler shaders and so on. Properties of the
	* object passed into this function are:
	*
	* - pass: value of {@link Layer#shaderPass} of the Layer being rendered.
	* - chunks: Object containing custom shader chunks that will replace default ones.
	* - customFragmentShader: Completely replace fragment shader with this code.
	* - forceUv1: if UV1 (second set of texture coordinates) is required in the shader. Will be
	* declared as "vUv1" and passed to the fragment shader.
	* - fog: the type of fog being applied in the shader. See {@link Scene#fog} for the list of
	* possible values.
	* - gamma: the type of gamma correction being applied in the shader. See
	* {@link Scene#gammaCorrection} for the list of possible values.
	* - toneMap: the type of tone mapping being applied in the shader. See {@link Scene#toneMapping}
	* for the list of possible values.
	* - ambientTint: the value of {@link StandardMaterial#ambientTint}.
	* - conserveEnergy: the value of {@link StandardMaterial#conserveEnergy}.
	* - occludeSpecular: the value of {@link StandardMaterial#occludeSpecular}.
	* - occludeDirect: the value of {@link StandardMaterial#occludeDirect}.
	* - shadingModel: the value of {@link StandardMaterial#shadingModel}.
	* - fresnelModel: the value of {@link StandardMaterial#fresnelModel}.
	* - cubeMapProjection: the value of {@link StandardMaterial#cubeMapProjection}.
	* - useMetalness: the value of {@link StandardMaterial#useMetalness}.
	* - blendType: the value of {@link Material#blendType}.
	* - twoSidedLighting: the value of {@link Material#twoSidedLighting}.
	* - diffuseTint: defines if {@link StandardMaterial#diffuse} constant should affect diffuse color.
	* - specularTint: defines if {@link StandardMaterial#specular} constant should affect specular
	* color.
	* - metalnessTint: defines if {@link StandardMaterial#metalness} constant should affect metalness
	* value.
	* - glossTint: defines if {@link StandardMaterial#shininess} constant should affect glossiness
	* value.
	* - emissiveTint: defines if {@link StandardMaterial#emissive} constant should affect emission
	* value.
	* - opacityTint: defines if {@link StandardMaterial#opacity} constant should affect opacity value.
	* - occludeSpecularFloat: defines if {@link StandardMaterial#occludeSpecularIntensity} constant
	* should affect specular occlusion.
	* - alphaTest: enable alpha testing. See {@link Material#alphaTest}.
	* - alphaToCoverage: enable alpha to coverage. See {@link Material#alphaToCoverage}.
	* - opacityFadesSpecular: enable specular fade. See {@link Material#opacityFadesSpecular}.
	* - alphaFade: fade value. See {@link Material#alphaFade}.
	* - sphereMap: if {@link StandardMaterial#sphereMap} is used.
	* - cubeMap: if {@link StandardMaterial#cubeMap} is used.
	* - ambientSH: if ambient spherical harmonics are used. Ambient SH replace prefiltered cubemap
	* ambient on certain platform (mostly Android) for performance reasons.
	* - useSpecular: if any specular or reflections are needed at all.
	* - fixSeams: if cubemaps require seam fixing (see {@link Texture#options.fixCubemapSeams}).
	* - emissiveEncoding: how emissiveMap is encoded. This value is based on Texture#encoding.
	* - lightMapEncoding: how lightMap is encoded. This value is based on on Texture#encoding.
	* - packedNormal: if normal map contains X in RGB, Y in Alpha, and Z must be reconstructed.
	* - forceFragmentPrecision: Override fragment shader numeric precision. Can be "lowp", "mediump",
	* "highp" or null to use default.
	* - fastTbn: Use slightly cheaper normal mapping code (skip tangent space normalization). Can look
	* buggy sometimes.
	* - refraction: if refraction is used.
	* - skyboxIntensity: if reflected skybox intensity should be modulated.
	* - useCubeMapRotation: if cube map rotation is enabled.
	* - useInstancing: if hardware instancing compatible shader should be generated. Transform is read
	* from per-instance {@link VertexBuffer} instead of shader's uniforms.
	* - useMorphPosition: if morphing code should be generated to morph positions.
	* - useMorphNormal: if morphing code should be generated to morph normals.
	* - reflectionSource: one of "envAtlasHQ", "envAtlas", "cubeMap", "sphereMap"
	* - reflectionEncoding: one of null, "rgbm", "rgbe", "linear", "srgb"
	* - ambientSource: one of "ambientSH", "envAtlas", "constant"
	* - ambientEncoding: one of null, "rgbm", "rgbe", "linear", "srgb"

[mvaligursky]

And also, if the option has moved to litOptions, we should generate some deprecated properties that warn user the option has moved. Those would ideally be added to deprecated.js

[GSterbrant]

I added an options class which is used to hookup deprecation callbacks, and replaced the use of an anonymous options object with this new (albeit empty) one.

[mvaligursky]

Looks good to me. Please do test it with at least engine examples to make sure all is happy.