backends: X11: Fix window size passed to event parser.

When SHM is not in use, the win->context field is not used to construct
the backend context and the width and height fields are uninitialized
which leads to wrong (or none) events from mouse pointer movement.

Fix this by using self->context which is correct in all cases.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>