source and destination format, cache those values in the newly added
SwsContext:srcFormatBpp and SwsContext:dstFormatBpp fields, and remove
the fmt_depth() function.


git-svn-id: file:///var/local/repositories/mplayer/trunk/libswscale@30419 b3059339-0415-0410-9bf9-f77b7e298cf2