Define a new Image operand bit mask for non constant offsets

Define a new Image operand bit mask for non constant offsets

For details refer to https://gitlab.khronos.org/spirv/SPIR-V/-/issues/639
As part of the commit following changes have been introduced:
1. Added a separate section in spirv xml to reserve vendor specific bit masks.
2. Added a new image operand bit mask to support non constant offsets in textureGatherOffsets as defined in GL_NV_gpu_shader5