Modify autogeneration script to pull data from gl_ext.xml and generate
entry_points_gles_2_0_ext_autogen.cpp/h as a replacement for
entry_points_gles_2_0_ext.cpp/h

Bug:angleproject:2263
Bug:angleproject:1309
Change-Id: Ie21079f8ec5f85c657b891f6d6d59306a4c3b5fe
Reviewed-on: https://chromium-review.googlesource.com/1005409
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>