OpenGL's pixel shader is built with rslc's builder as the other
backends' are, and the text generator goes. rslc is told what the
context's header said: framebuffer fetch by the EXT or ARM extension,
conservative depth in place of early fragment tests, and subgroup
reductions.
Built for random pixel uids against GLSL 4.50 with subgroups, 3.30 with
conservative depth, and ES 3.10 and 3.20 with either framebuffer fetch,
each stage these contexts can be asked for compiles in glslangValidator.