toSICoordinate method
Transform a point from the coordinate system of the container into the ScalableImage's coordinate system. This method adjusts for the origin of the SI's viewport.
Implementation
Offset toSICoordinate(final Offset c) => Offset(
(c.dx - translateX) / scaleX + siViewport.left,
(c.dy - translateY) / scaleY + siViewport.top);