Adds a new pix fmt function that selects the best dest fmt from among 2 formats to convert to, given a certain src format.

Similar to avcodec_find_best_pix_fmt(), but instead only compares two destination
pix fmts and selects one of these fmts as the best dest format when converting from
a given source format. Also, as opposed to avcodec_find_best_pix_fmt() which supports
only the first 64 defined pixel formats, this supports as input any of the defined
pixel formats.

Also replaces the old function to rely on the new function.
Signed-off-by: Matthew Einhorn <moiein2000@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>