glsl: Enable ARB_fragment_coord_conventions functionality in GLSL 1.50.

glsl: Enable ARB_fragment_coord_conventions functionality in GLSL 1.50.

GLSL 1.50 incorporates the functionality of the
ARB_fragment_coord_conventions extension, so we need to make this
functionality available even if the extension isn't enabled.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>