vp8dec: use is_alt_data option to prevent timestamp collisions

vp8dec: use is_alt_data option to prevent timestamp collisions

altref/invisible frames usually stored in container with same timestamp as
dependet frame. This make basevideodecoder to update timestamp for dependet
frame and couse TS colision on next frame:

                 ^- here is altref
time     : 1 2 3 4 5 6 7 8 9
webm ts  : 1   3 5 5   7   9
vp8dec ts: 1   3   7   7   9

Fix bug: https://bugzilla.gnome.org/show_bug.cgi?id=655245

Signed-off-by: Oleksij Rempel (Alexey Fisher) <bug-track@fisher-privat.net>