Fix few issues with Metal on iOS

[frenzibyte]

While trying to run Metal on iOS, I've came across a list of issues all of which are resolved in this PR. I've separated each fix to its own commit.

• 7f1d7c8: Similar to fa6481f, CGRect is too large to use objc_msgSend for, therefore objc_msgSend_stret is used. That being said, objc_msgSend_stret is not available on ARM64 devices, so both are used depending on the process architecture.
• 6a7ab9e: Loading native library on iOS is not supported, and not passing a metaclass pointer at all doesn't look to break anything as far as I've checked, therefore I suppose it's fine to keep it null?
• c136871: Applying a native scale to CAMetalLayer.drawableSize causes the application to actually be descaled by the same factor instead. Removing the scale factor fixes the issue, and macOS doesn't do it at all in the first place.