Re #1750:
 - Replaced "enum CodecID" with "unsigned" (previous fix raised compile error for some older ffmpeg versions).
 - Handled PIX_FMT_GBR24P hassle.



git-svn-id: http://svn.pjsip.org/repos/pjproject@4808 74dad513-b988-da41-8d7b-12977e46ad98