TexImage, SubImage and the Compressed variants were all taking a GL
Context as the first parameter, which is a layering violation and
also caused problems with reworking how the sync works. Fix this
by refactoring them in the same style as the CopyTex* entry points.

BUG=angleproject:1260
BUG=angleproject:747

Change-Id: Ibe5e87d0ebc790e2dcadb8ba153cf40fec73d1f6
Reviewed-on: https://chromium-review.googlesource.com/327258Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>