Wrap modes for integer textures need to be implemented in shaders in
HLSL. This requires more complex transformations on the texture
coordinates, so store the texture coordinate code for each texture
coordinate in a string variable.

BUG=angleproject:1244
TEST=dEQP-GLES3.functional.shaders.texture_functions.* (no regression),
     dEQP-GLES3.functional.texture.units.* (no regression)

Change-Id: Iadd9617a7f906aa1d56dabfba6639f932152e6f1
Reviewed-on: https://chromium-review.googlesource.com/336637Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>