mplayer2fb: use av_frame_*() instead of deprecated functions

Use av_frame_*() instead of deprecated avcodec_*_frame() functions.
mplayer2fb: drop checks for avcodec versions

Drop checks for avcodec versions.
libmpdemux: drop avformat version check

Drop avformat version check.
mplayer: fix a compiler warning with libass is disabled

attachment_is_font() is only needed when CONFIG_ASS is defined.
demux_mpg: fix a compiler warning

Fix a compiler warning on 64-bit targets.
dec_sub: fix a compiler warning

Fix a compiler warning when CONFIG_ASS is not defined.