Remove the "override" table. That gets in the way of some solutions.
If a format cannot be supported by a native Vulkan format as indicated
in the "map" table, then check fallbacks.

Also add support for native RGBA4 and R5G5B5A1 support. Previously
those formats would be emulated with RGBA8 due to the override, but
now code will check if the native format is available and use it.

Bug: angleproject:4282
Change-Id: Ib33ea40543d91a2c2a95075b277f825a8822037c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1994538Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>