Nir: add support for gather offsets

Karol Herbst requested to merge karolherbst/mesa:nir_tg4 into master

values inside the offsets parameter of textureGatherOffsets are required to be constants in the range of [GL_MIN_PROGRAM_TEXTURE_GATHER_OFFSET, GL_MAX_PROGRAM_TEXTURE_GATHER_OFFSET]. As this range is never outside [-32, 31] for all existing drivers inside mesa, we can simply store the offsets as a int8_t[4][2] array inside nir_tex_instr.

Right now only Nvidia hardware supports this in hardware, so we can turn this on inside Nouveau for the NIR path as it is already enabled with the TGSI one.

Edited by Karol Herbst

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