[Breaking] Quad render class, drawQuadWithShader refactor, Camera render events moved.

[mvaligursky]

Breaking change

• camera.onPreRender and onPostRender used to be called inside the render pass (during rendering to camera's render target), but that would not allow another render pass to execute within those callbacks. The callbacks were moved to execute just before and right after the render pass, allowing use of different render targets. Internally, only cubemap-renderer is affected, as used in RenderCubemap example - it does cubemap postprocessing inside the onPostRender callback.

Other changes

• new class, QuadRender - renders a quad using the Shader, supports uniform buffers, and does not handle RenderPass set up. Existing drawQuadWithShader now uses it internally. Being able to render multiple quads within a single render pass will speed up WebGL rendering on tiled architecture, allowing some systems to render those inside a single render pass, instead of multiple. For example cookie texture rendering for clustered, and texture based morphing with many targets.
• GraphicsDevice now owns a copy of quad vertex buffer. We don't create one for drawQuadWithShader, nor one for each post-processing effect instance.
• RenderPass now has a before function, allowing things to execute before the pass (in addition to existing after)
• WebGlGraphicsDevice has a simple copy of quadWithShader for its internal purposes, as drawQuadWithShader has moved out of graphics to a scene level, to work with shader processing / shader library, and its not a core graphics functionality as well.
• fix to MorphInstance to not update normal render target if normals are not allocated (newly added Assert triggered for this issue)
• extracted processShader function from material, to be able to use it for other shader processing (QuadRender)