mesa: Allow sampling from units >= MAX_TEXTURE_UNITS in shaders.

mesa: Allow sampling from units >= MAX_TEXTURE_UNITS in shaders.

The total number of units used by a shader is limited to MAX_TEXTURE_UNITS,
but the actual indices are only limited by MAX_COMBINED_TEXTURE_IMAGE_UNITS,
since they're shared between vertex and fragment shaders.

NOTE: This is a candidate for the 7.11 branch.

Signed-off-by: Henri Verbeet <hverbeet@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>