avcodec/v4l2_buffers: read height/width from the proper context

avcodec/v4l2_buffers: read height/width from the proper context

Frames are generally dequeued into capture buffers, so using
the output properties here was incorrect. It happened to work
fine for decoding, since the output/capture buffers have the same
dimensions.

For the v4l2 scaler, the dimensions can be different between output
and capture. Using the buffer's associated context makes this code
work correctly regardless of where the frame is coming from.

Signed-off-by: Aman Gupta <aman@tmm1.net>