vaapipostproc: fix output buffer WxH for crop meta forwarding

vaapipostproc: fix output buffer WxH for crop meta forwarding

Adding crop meta x,y to w,h only compensates for left,top
cropping.  But we also need to compensate for right,bottom
cropping.

The video meta contains the appropriate w,h (uncropped)
values, so use it instead.

Test:

 gst-launch-1.0 -vf videotestsrc num-buffers=500 \
  ! videocrop top=50 bottom=30 left=40 right=20 \
  ! vaapipostproc ! vaapisink & \
 gst-launch-1.0 -vf videotestsrc num-buffers=500 \
  ! videocrop top=50 bottom=30 left=40 right=20 \
  ! vaapipostproc ! identity drop-allocation=1 \
  ! vaapisink