Fix regression brought by ce67728a
As stated in the 'BIG FAT WARNING', decoder_t and filter_t first members must
be the same ones.

This solves skins2 numerous images failing to be displayed due to swscale
 video filter no longer working properly (a filter_t to decoder_t cast issue)

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

(I was wondering why those kept being recompiled)

This will make variable inheritance to child inputs much simpler

No need to do that check anymore.

and refactor playlist code for optimization

Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>

Please, be careful to NOT commit such hacks.