This patch is the first one in the series of supporting GLSL texture
function textureGather/textureGatherOffset on D3D11.

According to ESSL 3.1 SPEC (Chapter 8.9.3, Page 130), the definition
of textureGather on sampler2D is:
    gvec4 textureGather (gsampler2D sampler, vec2 P[, int comp])

The parameter "comp" is optional, and if it is specified, the value
of "comp" must be a constant integer expression with a value of 0, 1,
2, or 3, identifying the x, y, z, or w postswizzled component of the
four-component vector lookup result for each texel, respectively. If
comp is not specified, it is treated as 0.

According to the definition above, textureGather is equivalent to
Texture2D.Gather[Red|Green|Blue|Alpha] in HLSL, where we can use a
switch-case expression to choose the right HLSL texture function.

The features listed here will be implemented in the following patches:
1. Support textureGatherOffset
2. Support textureGather[Offset] on isamplers and usamplers
3. Support textureGather[Offset] on textures when swizzle is on
4. Support textureGather[Offset] on shadow samplers

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

Change-Id: Iff2ed4f8b65dad613cb0bafdfd19f8f0528e832c
Reviewed-on: https://chromium-review.googlesource.com/1232980
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>