Angle support on Apple platforms

[kvark]

This is not very useful given the good state our Metal backend is in. But it would be interesting nonetheless, to help with bugs, to allow people develop GLES backend on macs, and even compare performance to some extent.

#2257 brings in some of that, but we can't pass correct window handle to Angle. It expects CALayer, but we only have NSView from RawWindowHandle - rust-windowing/raw-window-handle#14

We can do the same thing we do on Metal backend - asking for CALayer and even creating it if needed.

[jinleili]

Preparing to compile angle to iOS, found that ANGLE does not have plans to support GLES on iOS.

[cwfitzgerald]

There's very little reason to follow through on this, so I'm going to close.

[teoxoy]

This was actually added by #2461. Not for iOS though.