This implements a NULL driver in OpenGL by stubbing out most of the
GL functions in FunctionsGL except a few static "Gets" that are needed
for initialization with Chromium and the tests. It is intended to be
used for performance testing ONLY and will not have correct behaviour.

It also adds a define to enable conditionally excluding the null entry
points for implementations that wish to save on a bit of binary size.

Also fixes some of the typedefs in functionsgl_typesdefs.h that were
turned up after implementing the direct assignment from NULL stub
entry point, generated from gl.xml, to the function pointer with type
defined from functionsgl_typedefs.h.

BUG=angleproject:2188

Change-Id: Ifa1e4739cb471ab6b52a4bf24c16d9eb4b334ac5
Reviewed-on: https://chromium-review.googlesource.com/727530
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>