This is partially complete - it is missing the auto-gen for custom
ANGLE and Chromium extensions that aren't present in gl.xml. We
can upstream some of these extensions, but will also need to make
custom handling for some extensions that are too volatile or under-
specced to upstream to Khronos.

This also tweaks some of the Context method names to match what the
auto-generator expects. It also updates some TexStorage entry points.

Also includes a specialized error return value for glTestFenceNV.

Also removes the TexImage3DOES entry point which was neither used
nor accessible to the application.

Bug: angleproject:2263
Change-Id: I3667c22b1b15f84ab4c7dfecb5745aaf73d11e76
Reviewed-on: https://chromium-review.googlesource.com/846420
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>