imgconvert: Implement avcodec_find_best_pix_fmt_of_list()

The old avcodec_find_best_pix_fmt() was insufficient due to 64 pix_fmt limit.
In ffmpeg this problem has been solved long ago through avcodec_find_best_pix_fmt2()
Today libav has added a incompatible modified version of avcodec_find_best_pix_fmt2()
under the same name, thus again breaking ABI/API ...

The avcodec_find_best_pix_fmt_of_list() added in this commit here makes the libav
API available to ffmpeg users too.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
