GdkWin32: Fix DND destination window search
This MR fixes two issues:
- DND not working when recording the screen e.g. with ScreenToGif, or in live screencasts during calls / chats with Skype, Google Meet etc.
- OLE2 DND not working when layered windows are suppressed in GTK3 (GDK_WIN32_LAYERED=0) and the DND indicator is placed right under the cursor. (That's not very important for GTK3, but it really is for GTK4)
Follow-up from !2767 (comment 1054295)
Edited by Luca Bacci