Merge pull request #120 from pattern-x/feature/fix-hotpoint

Fix a bug that it cannot add hotpoint successfully

public/demo/dxf_6_hotpoints.html

@@ -174,7 +174,7 @@
             }
 
             // create hotpoint...
-            if (addingHotpoint && target instanceof HTMLCanvasElement) {
+            if (addingHotpoint && (target instanceof HTMLCanvasElement || target.classList.contains("css2d-renderer"))) {
                 const location = viewer.getHitResult(event);
                 if (!location) {
                     return;