Fixes handling of packed vertex formats, adds new overrides for 32-bit
[SU](NORM|SCALED) vertex types, and handles half-precision floats
correctly.

Pixel 2 does not natively support certain 10-10-10-2 vertex formats;
this change does not add support for them.

Test: ./angle_deqp_gles3_no_gtest --deqp-egl-display-type=angle-vulkan -n 'dEQP-GLES3.functional.vertex_arrays.*'
Test: ./angle_end2end_tests --gtest_filter='AttributeLayout*/ES3_Vulkan'
Test: ./angle_end2end_tests --gtest_filter='VertexAttribute*/ES3_Vulkan'
Bug: angleproject:3193
Change-Id: I5ae4edd743e86e3e89e2697034c04dc4d9ecd1f9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1668230Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: James Dong <dongja@google.com>