This is a refactoring change only. It will facilitate moving these
functions to a shared library. A shared library will allow us to more
easily override GL functions with an interception layer. The intercept
layer will allow us to do emulation for features like offscreen tests.

Bug: angleproject:4845
Change-Id: I0e00c0c22a26f1bb8faec30a825c71953033f8a9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2305039
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>