glsl: Add gl_uniform_buffer_variable::IndexName field

glsl: Add gl_uniform_buffer_variable::IndexName field

glGetUniformIndices requires that the block instance index not be
present in the name of queried uniforms.  However,
gl_uniform_buffer_variable::Name will include the instance index.  The
IndexName field is added to handle this difference.

Note that currently IndexName will always point to the same string as
Name.  This will change soon.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Carl Worth <cworth@cworth.org>