Implement a workaround for widespread bugs calling glReadPixels with
RGBA/UNSIGNED_SHORT against R16/RG16 color attachments. Read back the
data using the GL_IMPLEMENTATION_COLOR_READ_FORMAT, and then rearrange
the read back pixels to fit the RGBA layout.

Also skip RGB16/RGB16_SNORM texture sample test on Nexus 5X/Nexus 6P
due to a another driver bug.

Bug: chromium:1000354, angleproject:4214, angleproject:4215, angleproject:4245
Change-Id: Iedea6f4136878cac5ad0dec3757c77b73502e1cd
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1952166Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Shrek Shao <shrekshao@google.com>