Crash on macOS when screen resolution changes when using SDL UI frontend
Description of problem:
In the above configuration, booting NetBSD works fine up to the point where the kernel sets the framebuffer resolution for the console, which results in a window size change. At this point, the OS terminates the qemu process with this error message:

```
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'NSWindow geometry should only be modified on the main thread!'
*** First throw call stack:
(
	0   CoreFoundation                      0x00000001849208c0 __exceptionPreprocess + 176
	1   libobjc.A.dylib                     0x0000000184419eb4 objc_exception_throw + 60
	2   CoreFoundation                      0x0000000184945bac _CFBundleGetValueForInfoKey + 0
	3   AppKit                              0x00000001880a6ab8 -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] + 240
	4   AppKit                              0x00000001880b2a38 -[NSThemeFrame _tileTitlebarAndRedisplay:] + 88
	5   AppKit                              0x00000001880c18a0 -[NSTitledFrame _titleDidChange] + 116
	6   AppKit                              0x0000000188a92f04 -[NSTitledFrame setTitle:subtitle:] + 420
	7   AppKit                              0x00000001880c1570 -[NSThemeFrame setTitle:] + 52
	8   AppKit                              0x000000018866e0fc -[NSFrameView _updateTitleProperties:animated:] + 44
	9   AppKit                              0x0000000188a85e98 -[NSThemeFrame _updateTitleProperties:animated:] + 156
	10  CoreFoundation                      0x00000001848a0780 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 148
	11  CoreFoundation                      0x00000001849349a8 ___CFXRegistrationPost_block_invoke + 88
	12  CoreFoundation                      0x00000001849348f0 _CFXRegistrationPost + 440
	13  CoreFoundation                      0x000000018486f434 _CFXNotificationPost + 764
	14  Foundation                          0x0000000185960c74 -[NSNotificationCenter postNotificationName:object:userInfo:] + 88
	15  AppKit                              0x000000018881da88 -[NSWindowTitleController _propertiesChanged:] + 128
	16  AppKit                              0x00000001880c1388 -[NSWindow _dosetTitle:andDefeatWrap:] + 156
	17  libSDL2-2.0.0.dylib                 0x0000000106aa9abc Cocoa_SetWindowTitle + 104
	18  qemu-system-aarch64                 0x0000000105006628 sdl_update_caption + 256
	19  qemu-system-aarch64                 0x0000000105007838 sdl_mouse_mode_change + 168
	20  qemu-system-aarch64                 0x00000001054ab100 notifier_list_notify + 36
	21  qemu-system-aarch64                 0x0000000104d28124 qemu_input_check_mode_change + 96
	22  qemu-system-aarch64                 0x0000000104e13a74 hid_pointer_activate + 32
	23  qemu-system-aarch64                 0x0000000104f44c2c usb_process_one + 464
	24  qemu-system-aarch64                 0x0000000104f4491c usb_handle_packet + 120
	25  qemu-system-aarch64                 0x0000000104f58a94 xhci_kick_epctx + 1888
	26  qemu-system-aarch64                 0x00000001052d8f78 memory_region_write_accessor + 264
	27  qemu-system-aarch64                 0x00000001052d8db8 access_with_adjusted_size + 348
	28  qemu-system-aarch64                 0x00000001052d8c04 memory_region_dispatch_write + 428
	29  qemu-system-aarch64                 0x00000001052e6cfc flatview_write_continue + 344
	30  qemu-system-aarch64                 0x00000001052e4068 flatview_write + 156
	31  qemu-system-aarch64                 0x00000001052e9424 subpage_write + 124
	32  qemu-system-aarch64                 0x00000001052d8db8 access_with_adjusted_size + 348
	33  qemu-system-aarch64                 0x00000001052d8c04 memory_region_dispatch_write + 428
	34  qemu-system-aarch64                 0x000000010532ebf4 io_writex + 184
	35  qemu-system-aarch64                 0x000000010532ed44 do_st_mmio_leN + 104
	36  qemu-system-aarch64                 0x0000000105323e78 do_st4_mmu + 536
	37  ???                                 0x0000000108a91750 0x0 + 4440266576
	38  qemu-system-aarch64                 0x00000001053108f0 cpu_tb_exec + 164
	39  qemu-system-aarch64                 0x0000000105311754 cpu_exec_loop + 1084
	40  qemu-system-aarch64                 0x0000000105310edc cpu_exec_setjmp + 48
	41  qemu-system-aarch64                 0x0000000105310dcc cpu_exec + 560
	42  qemu-system-aarch64                 0x0000000105332650 tcg_cpus_exec + 44
	43  qemu-system-aarch64                 0x0000000105332c1c mttcg_cpu_thread_fn + 240
	44  qemu-system-aarch64                 0x00000001054a7494 qemu_thread_start + 128
	45  libsystem_pthread.dylib             0x00000001847cf034 _pthread_start + 136
	46  libsystem_pthread.dylib             0x00000001847c9e3c thread_start + 8
)
libc++abi: terminating due to uncaught exception of type NSException
```

I think there have been other bugs of a similar nature in the past with the Cocoa UI. The regression may be because of stricter checks in the new macOS version.
Steps to reproduce:
1. Start qemu (the QEMU_EFI.fd is from Tianocore EDK2).
2. Wait for the NetBSD kernel to set framebuffer resolution and observe the crash.

With `-nographic`, the problem does not occur.