Fix extent position in flash.display.Graphics.

Minimum `x` and `y` values should take line thickness into account.

Example code: 

```haxe
import flash.display.Shape;

class TestCircle extends Shape {
  inline static var HEIGHT = 140;
  inline static var WIDTH = 120;

  public function new() {
    super();

    var g = this.graphics;
    g.lineStyle(5, 0xff0000);
    g.drawEllipse(-WIDTH, -HEIGHT, WIDTH * 2, HEIGHT * 2);
  }
}
```

In the current state, the ellipse will be cropped at the top and left edges.