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.

Re-land with a fix to typo in include guard.

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

Change-Id: I57c1ec1950fa05bd16275ca578eb5ee99b34a5ae
Reviewed-on: https://chromium-review.googlesource.com/339180Reviewed-by: Jamie Madill <jmadill@chromium.org>