Fix the rules for determining whether a blur image filter is valid (flutter/engine#43791)

See https://github.com/flutter/flutter/issues/130318

engine/src/flutter/display_list/effects/dl_image_filter.h

@@ -229,11 +229,15 @@ class DlBlurImageFilter final : public DlImageFilter {
   static std::shared_ptr<DlImageFilter> Make(SkScalar sigma_x,
                                              SkScalar sigma_y,
                                              DlTileMode tile_mode) {
-    if (SkScalarIsFinite(sigma_x) && sigma_x > SK_ScalarNearlyZero &&
-        SkScalarIsFinite(sigma_y) && sigma_y > SK_ScalarNearlyZero) {
-      return std::make_shared<DlBlurImageFilter>(sigma_x, sigma_y, tile_mode);
+    if (!SkScalarIsFinite(sigma_x) || !SkScalarIsFinite(sigma_y)) {
+      return nullptr;
     }
-    return nullptr;
+    if (sigma_x < SK_ScalarNearlyZero && sigma_y < SK_ScalarNearlyZero) {
+      return nullptr;
+    }
+    sigma_x = (sigma_x < SK_ScalarNearlyZero) ? 0 : sigma_x;
+    sigma_y = (sigma_y < SK_ScalarNearlyZero) ? 0 : sigma_y;
+    return std::make_shared<DlBlurImageFilter>(sigma_x, sigma_y, tile_mode);
   }
 
   std::shared_ptr<DlImageFilter> shared() const override {

engine/src/flutter/display_list/effects/dl_image_filter_unittests.cc

@@ -209,6 +209,23 @@ TEST(DisplayListImageFilter, BlurBounds) {
   TestBounds(filter, input_bounds, expected_output_bounds);
 }
 
+TEST(DisplayListImageFilter, BlurZeroSigma) {
+  std::shared_ptr<DlImageFilter> filter =
+      DlBlurImageFilter::Make(0, 0, DlTileMode::kMirror);
+  ASSERT_EQ(filter, nullptr);
+  filter = DlBlurImageFilter::Make(3, SK_ScalarNaN, DlTileMode::kMirror);
+  ASSERT_EQ(filter, nullptr);
+  filter = DlBlurImageFilter::Make(SK_ScalarNaN, 3, DlTileMode::kMirror);
+  ASSERT_EQ(filter, nullptr);
+  filter =
+      DlBlurImageFilter::Make(SK_ScalarNaN, SK_ScalarNaN, DlTileMode::kMirror);
+  ASSERT_EQ(filter, nullptr);
+  filter = DlBlurImageFilter::Make(3, 0, DlTileMode::kMirror);
+  ASSERT_NE(filter, nullptr);
+  filter = DlBlurImageFilter::Make(0, 3, DlTileMode::kMirror);
+  ASSERT_NE(filter, nullptr);
+}
+
 TEST(DisplayListImageFilter, DilateConstructor) {
   DlDilateImageFilter filter(5.0, 6.0);
 }