ARB_gpu_shader5: add compiler tests for textureGather()
ARB_gpu_shader5: add compiler tests for textureGather(.., comp)
textureGather: add support for selecting component
arb_gpu_shader5: add test to require textureGather comp be constexpr

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
arb_gpu_shader5: add compiler test for new textureGatherOffset variants
arb_gpu_shader5: new sigs textureGatherOffset comp