Fixed bug where resizing right and bottom edges on controls did not snap properly.

The bug was introduced in a combination of code modification and merge conflict resolved incorrectly.
For details see history for SnaplinePlacementBehavior.cs, in particular the following:
https://github.com/icsharpcode/SharpDevelop/commit/c825585043caa94fe741a7970728c160fa48fed1
https://github.com/icsharpcode/SharpDevelop/commit/b43ed8ab81079ab2298ac3ad52f1d60e7395cfe8