started big rework in top of scatter.

After bigger analysis, scatter is the best option if we want multitouch
features like zooming.
fix center_on when scale is changed
cap the execution of the downloader in time

This prevent to force loading all tile when zoom in/out. Much much
smoother.
when scaling down, improve the zoom to have a better quality
implement nice fading animation when a tile load, and preserve back tiles as well unless they are going out of the bounds
separate scatter to have a correct way to position/size the widget itself. And re-add Stencil.
first pass to try fixing the touch interactions
fix latests issues with zooming