Introduce angle::Format enum. 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/365413 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>