Copy schematic sheet pin's position during painting to avoid repeated…

… modifications. Fixes #19.

src/kicad/common.ts

@@ -88,6 +88,14 @@ export class At {
             this.unlocked = parsed.unlocked ?? this.unlocked;
         }
     }
+
+    copy() {
+        const at = new At();
+        at.position = this.position.copy();
+        at.rotation = this.rotation;
+        at.unlocked = this.unlocked;
+        return at;
+    }
 }
 
 export const PaperSize = {

src/viewers/schematic/painter.ts

@@ -551,7 +551,7 @@ class SchematicSheetPainter extends SchematicItemPainter {
         if (layer.name == LayerNames.label) {
             for (const pin of ss.pins) {
                 const label = new schematic_items.HierarchicalLabel();
-                label.at = pin.at;
+                label.at = pin.at.copy();
                 label.effects = pin.effects;
                 label.text = pin.name;
                 label.shape = pin.shape;