mesa: validate sampler uniforms during gluniform calls

mesa: validate sampler uniforms during gluniform calls

Patch fixes 'glsl-2types-of-textures-on-same-unit' in WebGL conformance
test suite. No Piglit regressions, fixes gl-2.0-active-sampler-conflict.

To avoid adding potentially heavy check during draw (valid_to_render),
check is done during uniform updates by inspecting TexturesUsed mask.

A new boolean variable is introduced to cache validation state.

v2: take into account case where 2 uniforms use same unit (curro)
    also do the check only when SSO is not in use, SSO has own
    path for sampler validation.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>