cinemaslides: Sigmoid math and off-by-one bug in levels usage

[wolfgangw]

In cinemaslides a sigmoid function is used to s-shape linear level progressions for fades and crossfades. Problem with that: The first and last values don't touch minimum and maximum (the amount by which they're off depends on the function's 'tuning'). This needs some shearing.

For the time being first and last are hardcoded to min and max.

Also there is a off-by-one bug in the loop that consumes these levels: Last value is not used, hence a fade out/crossfade doesn't land at -100 (which is, for now, hardcoded. See above). Not a real problem for fade in, of course.