Add support for pack/unpackUnorm2x16 to public SkSL.

This includes compile-time optimization and tests.

The unit test is disabled in a followup CL
(http://review.skia.org/447057) because it exposes a Radeon 5300M bug
in OpenGL.

Change-Id: I8b2f0411358aeb68c4edfeb0bd7a2814c4be1f40
Bug: skia:12202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447056
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
9 files changed