glsl: add forgotten textureOffset function for sampler2DArrayShadow

glsl: add forgotten textureOffset function for sampler2DArrayShadow

This was part of EXT_gpu_shader4 - as such it should have been supported
by glsl 130.
It was however forgotten, and not added until glsl 430 - with the wrong
syntax no less (glsl 430 mentions it was overlooked).
glsl 440 (but revision 8 only) fixed this finally for good.
At least nvidia supports this with just version glsl version 1.30 as well
(the spec doesn't explicitly say it should be supported retroactively),
so just add this to the other glsl 130 textureOffset functions.

Passes a (hacked) piglit tex-miplevel-selection test (2DArrayShadow
textureOffset -auto) with llvmpipe.

v2: fix up comment (by Ian), add testing to commit message.

Reviewed-by: Dave Airlie <airlied@gmail.com>