apple: fix crash when video_threaded is enabled with the Cocoa/Metal driver

[vanreece]

Summary

Enabling video_threaded = true on macOS crashes RetroArch immediately with the
Cocoa/Metal driver.

The threaded-video worker thread runs graphics-driver init, which calls into the
Cocoa UI layer — -[CocoaView setViewType:] and -[CocoaView setVideoMode:] —
and mutates NSWindow/NSView state off the main thread. AppKit forbids that,
so the process aborts ("NSWindow should only be modified on the main thread").

A plain dispatch_sync to the main queue is not enough on its own: during
CMD_INIT the main thread is parked in scond_wait() inside
video_thread_wait_reply(), so it never drains the main queue and the dispatch
would deadlock.

Fix

Two halves, both required:

• gfx/video_thread_wrapper.c — for CMD_INIT on Apple platforms, wait by
  pumping the main run loop in short slices (CFRunLoopRunInMode, 2 ms) and
  polling for the reply, instead of scond_wait(). All other commands keep the
  original lightweight wait, so there is no change off the init path.
• ui/drivers/ui_cocoa.m — -[CocoaView setViewType:] and
  -[CocoaView setVideoMode:] bounce to the main thread when invoked off it;
  combined with the pump above, the dispatch_sync completes instead of
  deadlocking.

Testing

Built on macOS (Apple Silicon, Metal). Before: video_threaded=true +
video_driver=metal crashes on load. After: launches and runs normally. The
default video_threaded=false path is unchanged — new code is gated to
CMD_INIT on __APPLE__.

🤖 Generated with Claude Code

[vanreece]

Sorry if AI generated PRs aren't welcome. Feel free to hate on it, but I thought it might be useful.

[LibretroAdmin]

If this actually works it would be a big deal - for a long time we had to simply 'stub out' threaded video because we couldn't figure out a way to make it work with Cocoa