WebGLRenderTargetCube: added fromEquirectangularTexture() method

[WestLangley]

As proposed in #15331 (comment).

Usage:

var options = {
	generateMipmaps: true,
	minFilter: THREE.LinearMipMapLinearFilter,
	magFilter: THREE.LinearFilter
};

var cubeMap = new THREE.WebGLRenderTargetCube( 512, 512, options ).fromEquirectangularTexture( renderer, texture );

I will provide docs in a later PR if we agree this is the API we want.

I will also port the examples.

[mrdoob]

Thanks!

[Mugen87]

I think we can update now the documentation and the TS file.

BTW: Some users were irritated of the fact that the project provides several ways to convert an equirectangular texture into the cube map format. It's good if we promote WebGLRenderTargetCube.fromEquirectangularTexture() as the way to go and remove EquirectangularToCubeGenerator.