fix unmatched save restore issue

Runtime/ui/painting/canvas.cs

         }
 
         public void restore() {
-            this._saveCount--;
-            this._recorder.addDrawCmd(new DrawRestore {
-            });
+            if (this._saveCount > 1) {
+                this._saveCount--;
+                this._recorder.addDrawCmd(new DrawRestore {
+                });
+            }
         }
 
         public int getSaveCount() {

Runtime/ui/painting/picture.cs

             });
         }
 
-        public Picture endRecording() {
-            if (this._states.Count > 1) {
-                throw new Exception("unmatched save/restore commands");
+        void restoreToCount(int count) {
+            count = this._states.Count - count;
+            while (count > 0) {
+                this.addDrawCmd(new DrawRestore());
+                count--;
+            }
+        }
+
+        void restore() {
+            var stateToRestore = this._getState();
+            this._states.RemoveAt(this._states.Count - 1);
+            var state = this._getState();
+
+            if (!stateToRestore.saveLayer) {
+                state.paintBounds = stateToRestore.paintBounds;
+            }
+            else {
+                var paintBounds = stateToRestore.paintBounds.shift(stateToRestore.layerOffset);
+                paintBounds = state.xform.mapRect(paintBounds);
+                this._addPaintBounds(paintBounds);
+        }
+        public Picture endRecording() {
+            this.restoreToCount(1);
             var state = this._getState();
             return new Picture(new List<DrawCmd>(this._drawCmds), state.paintBounds, this._isDynamic);
         }
                 }
 
                 case DrawRestore _: {
-                    var stateToRestore = this._getState();
-                    this._states.RemoveAt(this._states.Count - 1);
-                    var state = this._getState();
-
-                    if (!stateToRestore.saveLayer) {
-                        state.paintBounds = stateToRestore.paintBounds;
+                    //check for underflow
+                    if (this._states.Count > 1) {
+                        this.restore();
-                    else {
-                        var paintBounds = stateToRestore.paintBounds.shift(stateToRestore.layerOffset);
-                        paintBounds = state.xform.mapRect(paintBounds);
-                        this._addPaintBounds(paintBounds);
-                    }
-
                     break;
                 }