mesa: Add support for GL_ARB_ubo's glGetActiveUniformName().

mesa: Add support for GL_ARB_ubo's glGetActiveUniformName().

This is like a stripped-down version of glGetActiveUniform that just
returns the name, since the other return values (type and size) of
that function are now meant to be handled with
glGetActiveUniformsiv().

Fixes piglit ARB_uniform_buffer_object/getactiveuniformname

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>