Fix copying images of different formats on Metal

[kvark]

Relevant test is "dEQP-VK.api.copy_and_blit.core.image_to_image.simple_tests.whole_image_diff_format"

Metal copying requires pixel formats to match exactly. Vulkan only requires the total bit sizes to match. Whenever we detect that the formats are actually different, we reinterpret the source image with the destination format.

Unfortunately, the test still fails. Trying to modify Warden test to check this gives me interesting results:

• casting R32Uint to Rgba8Uint ends up filling only R component (d'oh, not expected)
• running via XCode debugger actually crashes in commit()...

[kvark]

I've spent quite a bit more time looking at this (than I'd hope to), and I consider this to be a candidate for Apple Metal upstream bug. I filed a private bug report, but we'll really need a simple repro case (written in ObjC/Swift) for them to investigate.