gl: Move glGetUniformLocation to shader compile time.

The lookup of the string names has significant overhead, which is why GL
gives you glGetUniformLocation so that you reference uniforms by
constant integers in your high performance path.

Reduces cairo-perf-trace runtime of firefox-planet-gnome by 1.06767% +/-
0.289265% (n=72) on my IVB macbook air.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>