Change FreeFormTool to pixel-based logic and add multipolygon factory

[simonasarvasova]

No description provided.

[Aiosa]

The degradation seems to be fixed, good work!

The problem with cropping still remains, e.g. if I create a polygon, zoom in so that it overflows the window, and modify the polygon, the invisibe parts gets removed. This could be solved for example by union operation when adding, deleting is bit harder, if original polygon A gets modifid to B, you would have to compute (A \ B) U B (or simply (A \ screen-viewport) U B). This works also for addition, but it might have problems on the borders of the window, since you get union of non-overlapping stuff that might be problamatic due to rounding error.

Anyway, please, try some boolean fix on this (making canvas big enough is unfortunately not an option). There is Greiner-Hormann clipping algorithm in the ext folder that could be used to remove the viewport part of the old polygon and put in the new modified part...