Fix Main Thread Checker warning on macOS

Cache off NSWindow's windowNumber in SDL_WindowData on setup and use that in `Cocoa_SendWakeupEvent` to prevent accessing windowNumber off the main  thread.

src/video/cocoa/SDL_cocoaevents.m

@@ -549,13 +549,11 @@ Cocoa_PumpEvents(_THIS)
 void Cocoa_SendWakeupEvent(_THIS, SDL_Window *window)
 { @autoreleasepool
 {
-    NSWindow *nswindow = ((__bridge SDL_WindowData *) window->driverdata).nswindow;
-
     NSEvent* event = [NSEvent otherEventWithType: NSEventTypeApplicationDefined
                                     location: NSMakePoint(0,0)
                                modifierFlags: 0
                                    timestamp: 0.0
-                                windowNumber: nswindow.windowNumber
+                                windowNumber: ((__bridge SDL_WindowData *) window->driverdata).window_number
                                      context: nil
                                      subtype: 0
                                        data1: 0

src/video/cocoa/SDL_cocoawindow.h

@@ -126,6 +126,7 @@ typedef enum
     @property (nonatomic) NSMutableArray *nscontexts;
     @property (nonatomic) SDL_bool created;
     @property (nonatomic) SDL_bool inWindowFullscreenTransition;
+    @property (nonatomic) NSInteger window_number;
     @property (nonatomic) NSInteger flash_request;
     @property (nonatomic) Cocoa_WindowListener *listener;
     @property (nonatomic) SDL_VideoData *videodata;

src/video/cocoa/SDL_cocoawindow.m

@@ -1615,6 +1615,7 @@ SetupWindowData(_THIS, SDL_Window * window, NSWindow *nswindow, NSView *nsview,
     data.nswindow = nswindow;
     data.created = created;
     data.videodata = videodata;
+    data.window_number = nswindow.windowNumber;
     data.nscontexts = [[NSMutableArray alloc] init];
     data.sdlContentView = nsview;