glsl/builtins: Use ivec for texel offsets in textureProjGradOffset.

glsl/builtins: Use ivec for texel offsets in textureProjGradOffset.

The GLSL 1.30 -> 4.10 specs all erroneously say "vec2" for a few
overloads of textureProjGradOffset, while most overloads and all other
texturing functions use ivec types.

The GLSL 4.20 specification corrects these to "ivec2", but doesn't
mention this as being a conscious change in behavior.  Nor does the
ARB_shading_language_420pack extension.  So presumably it was a typo.

At any rate, our builtin functions all use ivec already, so the fact
that these prototypes use plain vecs will only lead to applications
dying in a fire when trying to use them.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>