avcodec/microdvddec: support non persistent italic marker '/'

Inspired from a patch by Michal Wazgird <mplayer@cyryl.net>

See https://lists.mplayerhq.hu/pipermail/mplayer-dev-eng/2014-November/072772.html
Fix standalone compilation of the apng decoder on x86.
Print a warning if vp6 is muxed into flv: The output is flipped.

Fixes ticket #4132.
lavf/apngdec: print currently unsupported in-stream tags in a more readable form

Also use length and not stream position

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: James Almer <jamrial@gmail.com>
lavc/mss1: fix mem leak in case of init failure

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavc/rv30: fix mem leak in case of init failure

Also replaced return -1 with return AVERROR(EINVAL)

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>