gl_x11: fail gracefully if selected FBconfig has no X visual

glXGetVisualFromFBConfig() specifies specifies that it can return NULL
if there is no associated X visual. Instead of crashing, let
initialization fail. I'm not sure if this is actually supposed to work
with a fallback visual (passing a NULL visual to vo_x11_config_vo_window
would just do this), but let's play safe for now.

Apparently this can happen when trying to use vo_opengl over a remote
X display.
mplayer: don't keep previous video frame with --force-window

When for example switching off the video stream, and --force-window is
used, forcefully reconfigure the VO. This will reset the size, and will
clear the window with black.

Needed some effort, because you don't always want to clear the window
on "discontinuity" points like going to a next file: this would
introduce some flicker.