Panning right distance calculation for "pointsToAnimate"

Fix distance calculation for panning animation when you are panning right. 

Previously, the code incorrectly subtracted the left ledge size from the X origin of the sliding view controller rather than using the ViewDeck's width minus the left ledge size.

This resulted in an incorrect number of "pointsToAnimate" which created a duration that was too fast and did not correspond properly to the orientation velocity.