scale method
Implementation
void scale(double width, double height) {
if (_extent.isEmpty) return;
final extentWidth = (_extent[1].x - _extent[0].x).abs();
final extentHeight = (_extent[1].y - _extent[0].y).abs();
var fX = width / extentWidth;
if (fX * extentHeight > height) {
fX = height / extentHeight;
}
_canvasWidth = (extentWidth * fX).ceil();
_canvasHeight = (extentHeight * fX).ceil();
_isScaled = true;
}