Revert "glsl: don't try to lower non-gl builtins as if they were gl_FragData"

Revert "glsl: don't try to lower non-gl builtins as if they were gl_FragData"

This reverts commit a37e46323c7e18bec4160f2f66847c10b7041dc1.

It broke the game Overlord such that it hung a GCN GNU. While I don't know
how the hang happened because of its randomness and gfx corruption precedes
it, many of the shaders contain this:

out vec4 FragData[gl_MaxDrawBuffers];