ScanPlot2D: Small improvements

• Image disappear if the canvas size is changed
• When changing the frame ID, the zoom/pan should stay the same
• The zoom is not working very well on my computer (maybe mouse wheel issue with some plateform)
• Add ScanPlot2d to the UI documentation
• The vertical auto-centering looks wrong somehow (the y position looks to be negative; there is a margin at the bottom, which is never used anyway the pan)