mesa: Allow overriding GLSL version with environment variable

mesa: Allow overriding GLSL version with environment variable

Override the context's GLSL version if the environment variable
MESA_GLSL_VERSION_OVERRIDE is set. Valid values for
MESA_GLSL_VERSION_OVERRIDE are integers, such as "130".

MESA_GLSL_VERSION_OVERRIDE has the same behavior as INTEL_GLSL_VERSION,
except that it applies to all drivers, not just Intel's. Since the former
supercedes the latter, this patch disables the latter.

Reviewed-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>