sd_ass: initialize structs for external tracks properly

sd_ass_create_from_track() did not zero-initialize the created struct
sh_sub, which could cause at least an assertion failure at exit. Fix.
sub: #include required ctype.h directly

Include required ctype.h directly instead of relying on
libavutil/common.h to provide it. FFmpeg removed that include; Libav
hasn't so far, but relying on it was not intentional.
vf_uspp: drop this filter

vf_uspp uses the Snow codec from libavcodec. Libav has dropped support
for this codec. Delete the filter.
libav: switch from CODEC_ID to AV_CODEC_ID

Libav has dropped support for old CODEC_ID_* definitions. Switch to
the new AV_CODEC_ID_* variants. Increase minimum required libavcodec
version to the one adding the AV_ variants.
Merge remote-tracking branch 'upstream/master'