Fix a performance regression in mypaint engine

The regression is very noticeable on Android devices, the trace showing which functions are the problems: https://imgur.com/VPZIP0r.png

The regression is caused due to MyPaintSensorPack::read() method being called for every single brushOutline call -- which makes everything slow. Unfortunately, due to the design and structure of classes it is is difficult to have a state which is cached, so this is the best hack I could come up with and it probably isn't even idiomatic with Lager's data flow pattern.