Fixed two issues with the sequencer:
* using blend modes with startstill / endstill in combination with IPOs
failed, since there was no room to store the composited result.
(It was stored into the same TStripElem thereby effectively disabling the
effect of the IPO)
If you have no idea, what this is all about:
A common case was: use a single PNG as a title, extrude and try to fade
in / out using IPOs.
* startstill / endstill are always displayed, so that one can change them
also on movie-strips and scene-strips.
87eaa637c5