Ensure that things build again, and instead use the Windows API to
acquire the screen dimensions (note: this may need to be scaled for
HiDPI, but since I do not own a WinTab-based device, I will need to
keep the dimensions as-is for now).
Also update the gdkdnd-win32.c code to use formats rather than targets.
https://bugzilla.gnome.org/show_bug.cgi?id=773299