new_grid: The Return of the snap_to_X functions.

new_grid:  The Return of the snap_to_X functions.
 We no longer assume that Snap always uses the visible ruler lines.
 If you want to snap to the grid, and ignore the users zoom scale, use SnapPref::SnapToGrid_Unscaled
 This fixes 2 (known) oversights: "snap region(s) to grid" and "regions whose start are left of the canvas edge".