Fix of stroke width an simple despiking

Hey Roel,

you added enough code, that a can code my ideas. Here is a patch for the stroke with. The calculations are similar to my 2nd sketch here: https://github.com/roelj/inklingreader/issues/3 

I simplified it just the way, that I'm calculating the perpendicular direction to the line from point 1 to 2 an than going that direction from point 2 for a length of c->pressure to find the edge of the outline. You may find a better place for defining previous_x and previous_y than i did.

The despiking I implemented is very simple but works fine for me.

Width stroke with and despiking the Inkling sketchs look really nice. Hand writing looks much more neutral. I would appreciate if you add a command line option to de/activate the stroke width feature and do set the stroke with. (although your factor of 2000 looks nice)