Fix `Cocoa_GetWindowDisplayIndex` failing and causing a catastrophic crash

With the introduction of this function, it is possible that for certain
monitor and window configurations, creating an SDL window will cause a
native crash.

```
Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x0000000000000050
Exception Codes:       0x0000000000000001, 0x0000000000000050
Exception Note:        EXC_CORPSE_NOTIFY

Termination Reason:    Namespace SIGNAL, Code 11 Segmentation fault: 11
Terminating Process:   exc handler [56627]

VM Region Info: 0x50 is not in any region.  Bytes before following region: 140737486737328
      REGION TYPE                    START - END         [ VSIZE] PRT/MAX SHRMOD  REGION DETAIL
      UNUSED SPACE AT START
--->
      VM_ALLOCATE              7fffffe75000-7fffffe76000 [    4K] r-x/r-x SM=ALI

Thread 0 Crashed::  Dispatch queue: com.apple.main-thread
0   libSDL2.dylib                            0x10247f665 SDL_UpdateFullscreenMode + 357
1   libSDL2.dylib                            0x10247ec70 SDL_CreateWindow_REAL + 1504
2   ???                                      0x111262de8 ???
3   ???                                      0x110c39fff ???
4   libcoreclr.dylib                         0x101fdf2a9 CallDescrWorkerInternal + 124
```

Tracking thread from our end: https://github.com/ppy/osu-framework/issues/5190
Regressed with: https://github.com/libsdl-org/SDL/pull/5573

In testing, the window would not find a valid screen if created
"hanging" off a primary display with a secondary display below it. In
checking why this was the case, the `display_centre` was being
calculated with a negative y origin, causing a final negative value
falling outside all display bounds:

```
SDL error log [debug]: display_centre.y = -1296 + 1296 / 2

SDL error log [debug]: Display rect 0: 0 0 2560 1440
SDL error log [debug]: Display rect 1: 2560 -625 1080 2560
SDL error log [debug]: Display rect 2: 0 1440 1728 1296
```

The method that was being used to find the current window using the frame
origin/size seems unreliable, so I have opted to replace it with with a
tried method (https://stackoverflow.com/a/40891902).

Initial testing shows that this works with non-standard DPI screens, but
further testing would be appreciated (cc @sezero / @misl6 from the
original PR thread).

src/video/cocoa/SDL_cocoawindow.m

@@ -2230,9 +2230,6 @@ int
 Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window)
 { @autoreleasepool
 {
-    NSRect displayframe;
-    SDL_Point display_center;
-    SDL_Rect sdl_display_rect;
     SDL_WindowData *data = (__bridge SDL_WindowData *) window->driverdata;
 
     /* Not recognized via CHECK_WINDOW_MAGIC */
@@ -2240,23 +2237,17 @@ Cocoa_GetWindowDisplayIndex(_THIS, SDL_Window * window)
         return 0;
     }
 
-    /*
-     Considering that we already have the display coordinates in which the window is placed (described via displayframe)
-     instead of checking in which display the window is placed, we should check which SDL display matches the display described
-     via displayframe.
-    */
-    displayframe = data.nswindow.screen.frame;
-    
-    display_center.x = displayframe.origin.x + displayframe.size.width / 2;
-    display_center.y = displayframe.origin.y + displayframe.size.height / 2;
-    
-    for (int i = 0; i < SDL_GetNumVideoDisplays(); i++){
-        SDL_GetDisplayBounds(i, &sdl_display_rect);
-        if (SDL_EnclosePoints(&display_center, 1, &sdl_display_rect, NULL)) {
-            return i;
-        }
+    NSArray *screens = [NSScreen screens];
+
+    int index = 0;
+    for (NSScreen *screen in screens) {
+        if (screen == data.nswindow.screen)
+            return index;
+
+        index++;
     }
-    SDL_SetError("Couldn't find the display where the window is attached to.");
+
+    SDL_SetError("Couldn't find the display where the window is attached.");
     return -1;
 }}