This patch implements the translation from GLSL texture function
textureGatherOffset into HLSL by using the optional "offset"
parameter of Texture2D.Gather[Red|Green|Blue|Alpha].

This patch also defines the implementation-dependent limit
MIN_PROGRAM_TEXTURE_GATHER_OFFSET and MAX_PROGRAM_TEXTURE_GATHER_OFFSET
on D3D11. According to MSDN, the valid range of "offset" used in
Gather should be [-32, 31].
https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/gather4-po--sm5---asm-

This patch also refactors OutputTextureGatherFunctionBody() so that
some redundant code can be removed.

BUG=angleproject:2826
TEST=dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d.rgba8.*
     dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d_array.rgba8.*
     dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d.rgba8.*
     dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d_array.rgba8.*
     (without texture_swizzle)

Change-Id: Id0f411257b64c8a97428f16b1a86950ec6d36e2f
Reviewed-on: https://chromium-review.googlesource.com/1237303Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>