Optimize setting uniforms

Setting uniforms can take up to 15% of our time in the profiler, which is pretty standard in a 3D engine. We can do better, by both optimizing setting them and using less of them (yes, that means packing things together).

CC #646

The issue names no files, tests, or entry points; begin by profiling the uniform-setting path and reviewing the related discussion in issue #646. Done means reducing the time spent setting uniforms and reducing uniform usage through packing, with profiling results showing an improvement.

Written by the indexing model from the issue text.