The GLenum was already mostly packed. There were just a few missing
values because of the exclusion of things like GL_QUADS and GL_POLYGON
from GLES.

Also update the PackedEnumMap initialization to take an intializer.
The initializer is a list of key/values which is much more robust to
changes in the packed map.

Improves draw call speed slightly as there is no conversion needed any
more for the mode enum.

Bug: angleproject:2966
Change-Id: Icae658272c6234f29335f6a57a63cf341cf5b2a0
Reviewed-on: https://chromium-review.googlesource.com/c/1346529
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>