Revert "Remove all KTX support"

Revert "Remove all KTX support"

This reverts commit ada12ab0557a1f540e775288e2ce44f64fcd8c7e.

Reason for revert: Google3 needs it:
https://test.corp.google.com/ui#id=OCL:142184832:BASE:142184975:1481839118985:32fde8ef

Original change's description:
> Remove all KTX support
>
> It is untested and unused.
>
> Change-Id: I010ff4ad942738f362d42a99af4edbbb1cb0cd71
> Reviewed-on: https://skia-review.googlesource.com/6142
> Commit-Queue: Leon Scroggins <scroggo@google.com>
> Reviewed-by: Mike Klein <mtklein@chromium.org>
> Reviewed-by: Robert Phillips <robertphillips@google.com>
>

TBR=mtklein@chromium.org,mtklein@google.com,robertphillips@google.com,scroggo@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I1ea2f9487eb2212efbfcc514122792b70c9e8737
Reviewed-on: https://skia-review.googlesource.com/6181
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>