Review of the alpha to coverage change for alpha tested surfaces:
- An alpha which is flat over the pixel (fwidth 0) and equal to the opacity
threshold got a coverage of 0.5, so e.g. opacity 1 on an opaque or DXT1
texture turned half see-through with multisampling. Without an alpha edge
in the pixel the plain test decides now: step(opacity, alpha).
- The shaders took the coverage path whenever they were built with
multisampling, also where alpha to coverage is off (the reflections pass)
or does nothing (the single-sampled model previews), and grew the cut-outs
there by about half a pixel. model_ubo gets an alpha_to_coverage flag, set
together with the GL state, and the shaders keep the plain alpha test
without it. The previews no longer turn alpha to coverage on. The flag
takes 4 of the 12 padding bytes after shadow_tone, so the std140 layout and
the size of the block stay the same.
- The shader validator never set iMultisampling, so it never compiled the
alpha to coverage branches. Its "features on" configuration uses the
default of 2 now, on desktop GL and GLES.
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