ffprobe.xsd: add packets_and_frames element definition

This allows validation of ffprobe XML output if -show_packets and
-show_frames are both set.

Signed-off-by: Stefano Sabatini <stefasab@gmail.com>
avdevice/xcbgrab: Fix offset support

Found-by: carl
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>