[RPi] resize video width & height

因為不能用 ffmpeg 了, 只能用 mplayer 和 avconv 了

看 video width & height

mplayer -really-quiet -ao null -vo null -identify -frames 0 VIDEOFILE

改 video width & height

avconv -i input.mp4 -s 640x480 output.mp4

參考
* How can I get the resolution (width and height) for a video file from a linux command line?
* Need to downsample video to a lower resolution