- Add proper GPU conversion routine when the source and destination
  format of vertex attribute is half float data type.
- Add an end2end test which utilizes half float GPU conversion path.

Bug: angleproject:3802
Test: angle_end2end_tests --gtest_filter=*VertexAttributeTest.HalfFloatBuffer*
Change-Id: Ia88a4984156f8967796bea3852c3cde714f2acab
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1824799Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>