Add ImageConverter class so that we can use hwa support for convertion

Now only a converter based on FFMpeg swscale is implemented. Maybe the
interfaces are not good. It's ready for using ipp
TODO:
1. if in_fmt to out_fmt is not supported by hwa, then use ff(in base
class?)
2. move ImageConverter from decoder to video thread. decoder only do the
decoding
only g++ 4.7.2 can't compile
misc