Change the correct pointer's shape when resizing window

Change the correct pointer's shape when resizing window

The request to resize a window originates from the client. The display
server forces the cursor to a double-arrow shape regardless of whether
it is over the window or not, until the resize is done. This is to
make sure the cursor keeps that shape even if it moves outside of
the current boundaries of the window. With multiple pointers we need
to know which one to change. This is done by passing the pos_id from
button press event that starts the resize all the way to
ds_window_start_resize(). Then it needs to be stored in the window
structure for use when it is time to restore the cursor.