mesa/st: Fix precompile misses on compat GL VSes writing to color outputs.

mesa/st: Fix precompile misses on compat GL VSes writing to color outputs.

In compat GL, the gl_FrontColor/BackColor gets clamped unless you
explicitly turn it off with ARB_color_buffer_float, and most apps doing
floating point color rendering are going to be using non-compat varyings
anyway instead of hoping that ARB_color_buffer_float exists.  Thus, guess
that the app will get clamping on the color outputs to reduce draw-time
recompiles.

Saves 60 VS recompiles on half-life-2.trace on freedreno (and similarly for
many other compat GL apps).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9509>