fix: mask disappeared when having nested mask filter on Flutter web HTML (#45166)

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This PR fixes: flutter/flutter#133443, related: flutter/flutter#58546

The original codes attempts to cache the css string but it causes bugs when encountering nested the same mask filter blur. We should also set `filter` properties when currentFilter == incoming mask filter using the cached css string, not just ignore it. 

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Author: Kingtous

lib/web_ui/lib/src/engine/canvas_pool.dart

@@ -934,6 +934,7 @@ class ContextStateHandle {
   }
 
   ui.MaskFilter? _currentFilter;
+  String? _currentFilterCss;
   SurfacePaintData? _lastUsedPaint;
 
   /// Currently active shader bounds.
@@ -1003,19 +1004,20 @@ class ContextStateHandle {
     }
 
     final ui.MaskFilter? maskFilter = paint.maskFilter;
-    if (!_renderMaskFilterForWebkit) {
-      if (_currentFilter != maskFilter) {
-        _currentFilter = maskFilter;
-        context.filter = maskFilterToCanvasFilter(maskFilter);
-      }
-    } else {
-      // WebKit does not support the `filter` property. Instead we apply a
-      // shadow to the shape of the same color as the paint and the same blur
-      // as the mask filter.
-      //
-      // Note that on WebKit the cached value of _currentFilter is not useful.
-      // Instead we destructure it into the shadow properties and cache those.
-      if (maskFilter != null) {
+    if (maskFilter != null) {
+      if (!_renderMaskFilterForWebkit) {
+        if (_currentFilter != maskFilter) {
+          _currentFilter = maskFilter;
+          _currentFilterCss = maskFilterToCanvasFilter(maskFilter);
+        }
+        context.filter = _currentFilterCss;
+      } else {
+        // WebKit does not support the `filter` property. Instead we apply a
+        // shadow to the shape of the same color as the paint and the same blur
+        // as the mask filter.
+        //
+        // Note that on WebKit the cached value of _currentFilter is not useful.
+        // Instead we destructure it into the shadow properties and cache those.
         context.save();
         context.shadowBlur = convertSigmaToRadius(maskFilter.webOnlySigma);
         // Shadow color must be fully opaque.

lib/web_ui/test/html/compositing/canvas_mask_filter_golden_test.dart

@@ -150,6 +150,38 @@ Future<void> testMain() async {
       await canvasScreenshot(rc, 'mask_filter_transformed_$browser',
           region: screenRect);
     });
+
+    test('multiple MaskFilter.blur in $browser', () async {
+      const double screenWidth = 300.0;
+      const double screenHeight = 300.0;
+      const ui.Rect screenRect =
+          ui.Rect.fromLTWH(0, 0, screenWidth, screenHeight);
+
+      ContextStateHandle.debugEmulateWebKitMaskFilter = isWebkit;
+      final RecordingCanvas rc = RecordingCanvas(screenRect);
+
+      final SurfacePaint paint = SurfacePaint()
+        ..maskFilter = const ui.MaskFilter.blur(ui.BlurStyle.normal, 5);
+      rc.save();
+      rc.drawCircle(const ui.Offset(150, 150), 100,
+          paint..color = const ui.Color(0xFFC8C800));
+      rc.restore();
+      rc.save();
+      rc.drawCircle(const ui.Offset(150, 150), 50,
+          paint..color = const ui.Color(0xFFC800C8));
+      rc.restore();
+      rc.save();
+      rc.drawCircle(
+          const ui.Offset(150, 150),
+          20,
+          paint
+            ..color = const ui.Color(0xFF00C8C8)
+            ..maskFilter = const ui.MaskFilter.blur(ui.BlurStyle.normal, 10));
+      rc.restore();
+
+      await canvasScreenshot(rc, 'multiple_mask_filter_$browser',
+          region: screenRect);
+    });
   }
 
   testMaskFilterBlur();