This fixes several issues with the Speed Control Effect:
* IPO curve-deletion resulted in non-working effect
* easy retiming only allowed enlarging of strips,
now shrinking is also possible (easy retiming:
use the right display handle of input strip and
shrink or enlarge, will make the speed of the
strip exactly fit the effect strip's length)
* frames after end-of-display of input strips can
now be accessed (which is necessary for fix#2)
=> just use easy retiming from now on, it's fun :)
6f87d03b3c