vo_xv: avoid clearing too much on resize

vo_xv: avoid clearing too much on resize

vo_xv set the "use_fs" parameter to vo_x11_clearwindow_part(). This
meant it always used the whole screen size to calculate the area to
clear. I can't see why overriding the vo->dwidth/dheight values would
ever be the right thing to do (if in fullscreen they should be set to
match that), so remove the use_fs parameter and always use the
dwidth/dheight values in the function. Also delete code drawing back
borders in vo_xv_draw colorkey. That should already happen in
vo_x11_clearwindow_part(); if it doesn't then things need to be fixed
anyway because colorkey code only ran in fullscreen mode (but borders
must work in window mode too).