This general enum can encapsulate different formats between
GL/Vulkan/D3D9/D3D11/etc so we can use them in common routines like
PackPixels. It also can help us get rid of the ANGLEX enums which
we use to represent internal formats not present in GL.

It is currently used for Textures/Renderbuffers/Surfaces, but can
also be extended in the future to cover things like vertex formats.
It mirrors something like a DXGI_FORMAT in D3D11 or VkFormat.

BUG=angleproject:1455

Change-Id: I467d7b36d8fc92bb45239d56b9243d06f4e29378
Reviewed-on: https://chromium-review.googlesource.com/365413Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>