Use packed enum for DrawElementsType.

The packing and unpacking take a few extra instructions. But it
completely obviates the need for any switches in the validation code.
Speed is slightly faster or the similar depending on the back-end.

Also add gl_angle_ext.xml to GL entry point generator inputs. This was
missing and would cause the code generation to miss certain changes.

Bug: angleproject:2985
Change-Id: I1ea41a71db71135000166ead8305ec42d22ff7b3
Reviewed-on: https://chromium-review.googlesource.com/c/1351729
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
diff --git a/scripts/run_code_generation.py b/scripts/run_code_generation.py
index 92cc20b..18d1bd7 100755
--- a/scripts/run_code_generation.py
+++ b/scripts/run_code_generation.py
@@ -99,6 +99,7 @@
         'inputs': [
             'scripts/entry_point_packed_gl_enums.json',
             'scripts/gl.xml',
+            'scripts/gl_angle_ext.xml',
         ],
         'script': 'scripts/generate_entry_points.py',
     },