Follow-up from "Draft: Framework changes needed for HDR"

The following discussion from !132 (merged) should be addressed:

• @harishkrupo started a discussion:

  Can avoid two texture lookups by replacing with:

                  vec2 uv = texture2D(tex1, v_texcoord).rg - vec2(0.5);
                  u = uv.x;
  		v = uv.y;

I'm creating this issue for the three texture lookup optimizations that could be done as proposed above and in !132 (merged).

Before these can land, I would like to ensure that they are all covered by CI first. I think XYUV is the missing piece in tests.

More things to do here:

• consolidate magic values: !132 (comment 771939)
• implement shader garbage collection for too-long-unused shaders