Make the OnMouseMove method abstract for BasePaintBrush.
Make the OnMouseMove method abstract for BasePaintBrush.

Every brush needs to implement this differently, so it doesn't make
sense to have a default implementation.

PintaProject/Pinta,Mailaender/Pinta,jakeclawson/Pinta,jakeclawson/Pinta,PintaProject/Pinta,PintaProject/Pinta,Fenex/Pinta,Mailaender/Pinta,Fenex/Pinta