Metal: support texture's incomplete image definitions. glTexImage*, glCopyImage* will copy data to the image at the respective index, then during draw call, the image data will be transferred to real Metal texture. Test done: MipmapTest.DefineValidExtraLevelAndUseItLater For implementation notes, see src/libANGLE/renderer/metal/doc/TextureDataCompleteness.md Bug: angleproject:2634 Change-Id: I0ca24c8beff2e109a794260c436985e9f4650d83 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1906609 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>