updateScalebyDelta method
Increment or decrement the scale by an additive delta value.
Implementation
void updateScalebyDelta(double delta, {Offset? focalPoint}) {
// added focalPoint param
focalPoint ??= Offset(canvasSize.width / 2, canvasSize.height / 2);
final newScale = _scale + delta;
_gsTopLeftOffset = newGsTopLeftOnScaling(_gsTopLeftOffset, focalPoint, _scale, newScale);
_scale = newScale;
markDirty();
}