- OES_vertex_half_float extension requires this patch
- Add end2end tests to verify OES_vertex_half_float extension

BUG=angleproject:3191
BUG=angleproject:3802
Test:
 angle_end2end_tests --gtest_filter=*VertexAttributeTest.HalfFloatClientMemoryPointer*
 3DMark Icestorm GT1

Change-Id: Ia597021a5ae6b4853ee1199989ec3f9cc23c7fac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1793354
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>