Refactor signal compressor to have better timing properties
- before: emits signals with time range [1.0...2.0] * interval
- after: emits signals with time range [0.5...1.5] * interval
Bascially, now it handles it much better when interval is around
10-20 ms. With the old version it caused KisCanvas2 to drop frames
and look ugly when the user pans the canvas.