Rust SSGI shaders are slower than HLSL

[h3r2tic]

#21 introduced Rust GPU ports of SSGI/SSAO shaders. They are currently slower than the HLSL counterparts (see #21 (comment)). Some of that is because USE_AO_ONLY turned into a uniform parameter which the compiler cannot const-fold, but it doesn't account for all of the difference. We should make sure USE_AO_ONLY and any other important constants are actually constant, and investigate what causes the rest of the difference.

Once this is fixed, let's switch over to using the Rust shaders.