This extension is core in GLES 3.2
This CL enables the extension, added 412 new dEQP tests for texture
buffer. 410 of these tests pass, but there are two tests that fail,
detailed in anglebug.com/4933

Also includes a fix for completeness checks.

Based on a CL by Jonah Ryan-Davis.

Bug: angleproject:3573
Bug: angleproject:4933
Change-Id: I45759d765c88c64f21b592eab01d910dc4bc8da3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2521239
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>