Apply root window coordinate optimizations to crossing events as well

Apply root window coordinate optimizations to crossing events as well

* src/xterm.c (xi_compute_root_window_offset_enter): New
function.
(handle_one_xevent): Apply root window crossing optimizations to
XI_Enter and XI_Leave events too.  Not strictly necessary but
since we apply them to motion events it would be consistent to
apply them to crossing events as well as recommended by the
ICCCM.