Implement v4l2 input size autodetection in v4l2_read_header().

Move check on frame size after the device is opened and after
device_try_init() is attempted. If the provided size value is 0x0,
perform a VIDIOC_G_FMT ioctl() on the device, which sets size to the
current settings.

Originally committed as revision 22971 to svn://svn.ffmpeg.org/ffmpeg/trunk
