glsl: add signatures for textureGatherOffsets()

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
i965: Add asserts to ensure that ir_tg4 offset arrays are lowered

We don't have a message that does 4 independent offsets; a lowering
pass needs to lower it to 4 normal gather4s before reaching this
point.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>