Filter improvement

In the course of selfless struggle to exterminate an elusive bug, filter module met with major two-lines change substantionally changed its logic in an uncertain situations, making this update a must.

In the meantime, several updates were introduced providing some speed increase.

Main GUI got several new keybingings for zoom 1:1, namely Ctrl+1 (like Macromedia Fireworks, hallowed by its name) and Ctrl+ALt+0 (like Adobe Photoshop).