Incidentally `-y` also collides with avconv global options.
Update x11grab to match and document the option.
CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
The feature is implemented using a transparent window and drawing
inside it a rectangle filling the whole window to highlight it.
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>