ShadowMaps

[VReaperV]

1. A singular shadowmap atlas for all shadowmaps, allocated in rectangular regions of tiles in compute
2. The id of the corresponding light is written into a stencil texture with 1 1-byte value for each shadowmap tile
3. When rasterising the shadowmaps, export the light id as stencil value, to be used in stencil test
4. Possibly high pressure on the rasteriser? Might need to split it into multiple draws - 256 draws (for an 8192x8192 shadowmap atlas with 64x64 tiles this would cover 8x8 tiles/512x512 texels) over the shadowmap with corresponding viewports, although 1 shadowmap may then straddle draw bounds (most likely for cascades). Frustum culling is likely to heavily reduce the amount of rasterisation for each shadowmap tile though, especially with triangle cull. Culling would have to export indices to multiple locations though (probably okay with good culling?) (surface/cluster cull wouldn't need to because it can be represented as a bitmask)