Check if the FS output color comes from an FS input. If so, don't tag
the shader as linear. See code comments for more details.
During testing I added extra counters to check the number of times
linear shaders were used to be sure we're not accidentally disallowing
too many shaders. Things looked good with our in-house mksReplay test
suite.
This fixes some OpenGL CTS test failures with llvmpipe.
Signed-off-by: Brian Paul <brianp@vmware.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/7489
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21340>