Use coordinates provided by DND messages if available

Use coordinates provided by DND messages if available

This avoids an extra sync, which matters when dropping onto
Emacs running over a slow connection.

* src/xselect.c (x_handle_dnd_message): New args
`root_window_coords', `root_x' and `root_y'.
* src/xterm.c (x_coords_from_dnd_message): New function.
(handle_one_xevent): Pass root window coordinates to
x_handle_dnd_message.
* src/xterm.h: Update prototypes.