Live updates while filtering

At the moment changing the filter sliders does not give any visual feedback, setting cache image to false lets the Image to ask the image provider a new one every time the source is changed (which is already done), as mentioned in the docs.

Next thing I wanted to do is update the image while dragging the sliders, not only after releasing them. Requesting a new image every time the slider value changes seems a bit too intense and the slider handle lags. I tried putting a Timer to cap the request at maximum one every 100ms and seems to be a bit better, but still not perfect... Would you be ok with such a solution (or even with the real live updates)? I made a dedicated branch for the timer thing

Otherwise the image filtering has to be moved to a different thread, either with QtConcurrent::run() or moving the object that does the job in a proper QThread. I can try to do that too if you don't like the above solutions 😉