Fix calculation of edge input window size in case the real window is too small.

Fix calculation of edge input window size in case the real window is too small.

This error resulted in warnings like
"pixman_region32_init_rect: Invalid rectangle passed"

In case the window is smaller than handle_size * 2 the resulting
edge window got a negative size. Prevent that by limiting the
handle size to half the respective edge length. This also
prevents the corner windows from overlapping in case the window
is too small.