visionOS: use black instead of white for the mouse overlay

White is easier to see and people notice the overlay showing up when mouse input is enabled

src/video/uikit/SDL_CurvedContentView.swift

@@ -167,7 +167,7 @@ internal struct SDL_CurvedContentView: View {
         .overlay {
             if mouseInputEnabled {
                 // This enables mouse motion events, but blocks hover location
-                Color.white
+                Color.black
                     .opacity(0.001)
                     .pointerStyle(.shape(Circle(), size: .zero))
             }