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>