This patch implements the translation of textureGather[Offset] on
integer and unsigned integer format textures on D3D11.

According to MSDN, Gather[Red|Green|Blue|Alpha] can be used on integer
and unsigned integer format textures with float texture coordinates:
https://docs.microsoft.com/en-us/windows/desktop/direct3dhlsl/dx-graphics-hlsl-to-gather

Currently the dEQP tests on TextureGather[Offset] on cube map textures
in integer or unsigned formats still cannot pass. This issue is under
investigation.

Bug: angleproject:2826
Test: dEQP-GLES31.functional.texture.gather.basic.2d.*
      dEQP-GLES31.functional.texture.gather.basic.2d_array.*
      dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d.*
      dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d_array.*
      dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d.*
      dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d_array.*

Change-Id: If049dc395fb17cd34dec902630e3a1154e5d4806
Reviewed-on: https://chromium-review.googlesource.com/c/1271926
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>