Fix: Correct arrow head fill position in measurements

The `fillPolygon` method in `screenCanvas.drawController.ts` was using
y-coordinates directly after transformation by `worldToTarget`. However,
other primitive drawing operations (like `drawLineScreen`, `drawArcScreen`,
`fillRectScreen`, `drawTextScreen`) effectively transform these y-coordinates
further using `canvasSize.y - y_coordinate` before passing them to the
HTML5 canvas context.

This inconsistency caused the fill of measurement arrowheads (drawn via
`fillPolygon`) to appear in a different position than their outlines
(drawn via `drawLine`).

This commit modifies `fillPolygon` to apply the same
`this.canvasSize.y - screenPoint.y` transformation to its y-coordinates,
aligning it with the predominant coordinate handling strategy in the
`screenCanvas.drawController` and resolving the arrow head fill issue.

Addresses your feedback related to issue #17.

Author: google-labs-jules[bot]

src/drawControllers/screenCanvas.drawController.ts

@@ -468,9 +468,9 @@ export class ScreenCanvasDrawController implements DrawController {
         this.context.beginPath();
         screenPoints.forEach((screenPoint, index) => {
             if (index === 0) {
-                this.context.moveTo(screenPoint.x, screenPoint.y);
+                this.context.moveTo(screenPoint.x, this.canvasSize.y - screenPoint.y);
             } else {
-                this.context.lineTo(screenPoint.x, screenPoint.y);
+                this.context.lineTo(screenPoint.x, this.canvasSize.y - screenPoint.y);
             }
         });
         this.context.closePath();