mesa: Add support for glGetUniformIndices().

mesa: Add support for glGetUniformIndices().

This is a single entrypoint that maps from a series of names to the
indices of those names within the active uniforms list.  Each index is
like glGetUniformLocation()'s return value, except that it doesn't
encode an array offset.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>