This change is pure refactoring, it does not introduce any functional
changes.

Separate texture function output into a helper class and further into
different helper functions to make the code more maintainable.

Some of the logic is simplified slightly by eliminating duplicate
cases and limiting the scope of variables where possible, but care
has been taken to preserve the exact same functionality as before.

BUG=angleproject:1349
TEST=dEQP-GLES3.functional.shaders.texture_functions.* (no regression)
     dEQP-GLES3.texture.* (no regression)

Change-Id: I5d81b842d693c0055890d5724eae6c105e454cd8
Reviewed-on: https://chromium-review.googlesource.com/337931Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>