glsl: Handle the binding qualifier for UBO variables.

glsl: Handle the binding qualifier for UBO variables.

layout(binding = N) is equivalent to calling glUniformBlockBinding(_,N).

This currently only handles the GLSL 1.40 case - no interface names, no
arrays of uniform blocks.  This is okay since we don't yet support GLSL
1.50, and don't expose ARB_shading_language_420pack in ES 3.0.

v2: Move into the other function; use binding, not constant_value.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Paul Berry <stereotype441@gmail.com>