Use flat enum for PrimitiveMode. 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>