Paint triangles when mouse is dragged

Changed mouse interaction so that triangles are painted when mouse is
dragged instead of when mouse is moved. When a triangle reaches full
opacity, the ‘direction’ of painting can be flipped so that a triangle
can be depleted of opacity/erased. This flip requires that the mouse
drag event stops (a mouse up event is received). This is enforced so
that you can achieve perfectly full opacity without having to ‘time’
the ending of your drag.