Snap all shape points to closest "grid point" when "snap to grid" option is enabled

[dacap]

Actually we should be able to have two different ways to draw lines/shapes when "snap to grid" mode is enabled:

The left one use snap to grid only for the first and last points of the line, the mode at the right side uses snap to grid for all points in the line.

Idea by grymmjack
Further discussion: https://community.aseprite.org/t/snap-to-center-of-the-square-on-grid-system/89