Jumpy relative mouse movement fix

[andyac]

Please look at my Mouseinterface.cs file at lines 702-718 to see how I fixed the grainy relative mouse movement.

Calculations for relative cursor position were redone - movement is now much smoother.

Mouseinterface.cs only has changes to relative cursor position and minimum pen click threshold but appears completely different due to whitespace autoformatting.

[Serfboreds]

Posted for my own reference to the following link with all credit going to you for fixing the mouse movements (obviously brandonlw gets all the credit for the stuff you didn't improve): https://github.com/Serfboreds/uDraw-Wii---Smoother-Pen

This is so much more usable now, and has made this one of the most tremendously successful hacks I've applied (and I've done many).