wxOSX: Relax DoDragDrop() constraints.

DoDragDrop() not assert 'It must be called in response to a mouse down or drag event.'.
But when the user start DnD something from a frame rendered by CEF or similar framework,
it should be perfectly valid.

src/osx/cocoa/dnd.mm

@@ -494,8 +494,21 @@ - (nullable id)pasteboardPropertyListForType:(nonnull NSPasteboardType)type
     if (view)
     {
         NSEvent* theEvent = (NSEvent*)wxTheApp->MacGetCurrentEvent();
-        wxASSERT_MSG(theEvent, "DoDragDrop must be called in response to a mouse down or drag event.");
-
+        // relax the constraint set for DoDragDrop().
+        // if the user start DnD something from a frame rendered by CEF or similar framework, it should be perfectly valid.
+        if (theEvent == nil){
+            NSPoint mouse_location = [NSEvent mouseLocation];
+            theEvent = [NSEvent mouseEventWithType:NSEventTypeLeftMouseDragged 
+                location:mouse_location
+                modifierFlags:0
+                timestamp: 0
+                windowNumber: [NSWindow windowNumberAtPoint:mouse_location belowWindowWithWindowNumber:0]
+                context:nil
+                eventNumber: 0
+                clickCount: 0
+                pressure: 1.0];
+        }
+
         gCurrentSource = this;
 
         DropSourceDelegate* delegate = [[DropSourceDelegate alloc] init];