Make `RenderBuffer` API more public

[adamkewley]

To benefit specific use-cases, like shadowmapping.

Currently, the internal graphics API exposes RenderTexture, which is what the majority of rendering code needs+uses. However, there are cases where downstream code wants something more custom than a color+depth combination. E.g. in shadowmapping, typically only the depth information is written during the shadow pass.

This will require reading into:

• https://www.khronos.org/opengl/wiki/Framebuffer_Object

But the integration point would be to enable downstream renderers to create RenderBuffers themselves and then attach them to a RenderTarget (framebuffer) as-required for the particular shader in question.