Use the Vision Pro headset native refresh rate

The headset actually has dynamically adjusting refresh rate, but 90Hz is closer to it's normal operating mode.

src/video/uikit/SDL_uikitmodes.m

@@ -281,7 +281,7 @@ bool UIKit_AddDisplay(bool send_event)
     mode.h = (int)size.height;
     mode.pixel_density = 2;
     mode.format = SDL_PIXELFORMAT_ABGR8888;
-    mode.refresh_rate = 60.0f;
+    mode.refresh_rate = 90.0f;
 
     display.natural_orientation = SDL_ORIENTATION_LANDSCAPE;