winwidget_resize: Do not call winwidget_get_geometry after XFlush

While calling winwidget_get_geometry(winwid, NULL); at this point
would help alleviate flashing issues that can occur when feh has
to render a window two times in a row, or renders the initial image
with a resolution that differs from the one that is needed to
accomodate the resize.

However, it would also break --scale-down in floating setups. As
flashing is less annoying, we do not call winwidget_get_geometry.
here.

Partially reverts 6af56d54

See also https://github.com/derf/feh/pull/462

Thanks to Awal Garg for several joint debugging sessions.