fix for codecs that can change resolution (DNxHD)

Some codecs can change resolutions, namely DNxHD. This is a fix for that. When the frame size changes a new sws_Context needs to be created along with a new buffer_size.

here is example quicktime that changes resolution from 720p -> 1080p
https://dl.dropboxusercontent.com/u/170952/resolution_changing.mov.zip