Ken's rework of mesa/st builtins to NIR means that we'll have more NIR
shaders with color output types that are mismatched with the render target
types.  Since this is behavior that GLSL doesn't require, add it as a
shader_info option so the driver can know that it needs to ignore the FS
output's base type in favor of the actual render target's.  This prevents
needing additional variants in several mesa/st paths (clear, pbo upload,
pbo download), given that the driver already has to handle the variants
for any TGSI being passed to it (from u_blitter, for example).

Reviewed-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>