Improve performance of rasterization

[MarcusAndreasSvensson]

The rasterization shader has many opportunities for improvement.

The largest performance hit right now is the loop which handles the color contribution from every splat in the tile.

Some great optimizations would involve effectively culling and early stopping of the loop, less global memory fetches and less branching.

[MarcusAndreasSvensson]

Big improvement since #6

[MarcusAndreasSvensson]

7