Nir: add support for gather offsets
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.