Add partial selection support about lorien

[mbrlabs]

Hey, thanks for already using it :)

Currently in order to select a brush stroke you will have to select both the first and the last point of the stroke. This is also why just clicking on a stroke won't work. This has been implemented like that by @fenix-hub mainly because it's really fast to do it like that (and it works pretty well i think). In order to implement partial selection you would have to check every point of every brush stroke. This can be improved by only testing the brush strokes currently on screen strating with the ones closest to the cursor however.

Certainly a useful thing to have but something for the future i think. I'll have to figure out how to best implement it performance and UX wise.

[mbrlabs]

I finally got around to start implementing this in 7344dd6. For now it's just the partial selection (no matter how you drag the selection rectangle). It's really much more usable like that!

Performance is actually really good. I only test strokes that are on screen and with those i do a "broad-phase" collision check first by checking if bounding boxes intersect. Only then do i actually check if individual points are in the seletion rectangle.

I will leave this issue open, since i want to eventually implement the traditional (whole stroke selected) selection method as well. But i'm unsure if the dragging from left/right is the right approch...it might confuse people because it makes the tool behave differently.

[mbrlabs]

Finally closing this, as i think the current partial selection support is sufficient