canvas: restore() should reflect that smask groups are finished when …

…stateStack is empty.

This fixes #12367. When we end the SMask group and stateStack.length is zero,
nothing updates this.current to reflect it.

src/display/canvas.js

@@ -1192,7 +1192,7 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
       groupCtx.clearRect(0, 0, groupCtx.canvas.width, groupCtx.canvas.height);
       groupCtx.restore();
     },
-    resumeSMaskGroup: function CanvasGraphics_endSMaskGroup() {
+    resumeSMaskGroup: function CanvasGraphics_resumeSMaskGroup() {
       // Resuming state saved by suspendSMaskGroup. We don't need to restore
       // any groupCtx state since restore() command (the only caller) will do
       // that for us. See also beginSMaskGroup.
@@ -1254,6 +1254,9 @@ var CanvasGraphics = (function CanvasGraphicsClosure() {
         this.pendingClip = null;
 
         this._cachedGetSinglePixelWidth = null;
+      } else {
+        // We've finished all the SMask groups, reflect that in our state.
+        this.current.activeSMask = null;
       }
     },
     transform: function CanvasGraphics_transform(a, b, c, d, e, f) {