metal: check if in the main thread when calling create_surface

[jinleili]

Connections
#2727

Description
On macOS and iOS platforms, calling UI APIs in non-ui thread will cause unexpected results. Tell the user exactly what the problem is by checking if it is in the main thread.

Also tried using Objective-C GCD to specify the main thread to execute the code, but view: *mut c_void is not a thread-safe object, and I don't think actively jumping threads is something wgpu should do.

Testing
Tested on macOS and iOS.

[cwfitzgerald]

We should add documentation to create_surface and this function it will panic if not called on the main thread.

[cwfitzgerald]

Thanks!