SCI: Add a more generic solution for the problem found in bug #3605269

We now accept signals from all channels if channel 15 is missing. This
is a very rare edge case, but our behavior is now somewhat closer to what
the original interpreter seems to be doing. Mordack's appearance animation
is now synced properly. The more generic workaround works for songs 1840
and 1843, but not for 1849, which is still problematic and we still resort
to manually changing its dataInc selector