implement layered rendering

[paroj]

this will allow rendering multiple cameras in one draw call. e.g. PSSM, cube mapping

using instancing with ext is preferable to introducing the geometry shader.
However the geometry shader is easier to integrate.

Can be used to do indexed viewports for stereo.

requires:

• TEX_TYPE_2D_ARRAY, TEX_TYPE_CUBE_MAP support in shadow renderer. ideally via Generalized compositors #1640
• passing camera array to shader via ACT_

references:

• SV_RenderTargetArrayIndex/ gl_Layer
• https://registry.khronos.org/OpenGL/extensions/ARB/ARB_shader_viewport_layer_array.txt
• https://developer.nvidia.com/gpugems/gpugems3/part-ii-light-and-shadows/chapter-10-parallel-split-shadow-maps-programmable-gpus
• https://learnopengl.com/Advanced-Lighting/Shadows/Point-Shadows
• https://kondrak.github.io/posts/2016-05-21-rendering-in-vr-using-opengl-instancing/