Normalize the translation column of the color matrix. (#161109)

`dart:ui` specifies that the translation column of the color matrix
should be in the range 0..255. Skia expects a normalized range between 0
and 1. Skwasm was not normalizing this before passing it to Skia, but
CanvasKit was. After writing a unit test, it appears that the HTML
renderer has the same problem, so I also fixed that as well.

This addresses flutter/flutter#159697

engine/src/flutter/lib/web_ui/lib/src/engine/html/color_filter.dart

@@ -312,7 +312,18 @@ class SvgFilter {
 
 SvgFilter svgFilterFromColorMatrix(List<double> matrix) {
   final SvgFilterBuilder builder = SvgFilterBuilder();
-  builder.setFeColorMatrix(matrix, result: 'comp');
+
+  /// Flutter documentation says the translation column of the color matrix
+  /// is specified in unnormalized 0..255 space. `feColorMatrix` expects the
+  /// translation values to be normalized to 0..1 space.
+  ///
+  /// See [https://api.flutter.dev/flutter/dart-ui/ColorFilter/ColorFilter.matrix.html]
+  final normalizedMatrix = List<double>.generate(
+    matrix.length,
+    (int i) => (i % 5 == 4) ? matrix[i] / 255.0 : matrix[i],
+    growable: false,
+  );
+  builder.setFeColorMatrix(normalizedMatrix, result: 'comp');
   return builder.build();
 }
 

engine/src/flutter/lib/web_ui/lib/src/engine/skwasm/skwasm_impl/filters.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:ffi';
 import 'dart:typed_data';
 
 import 'package:ui/src/engine.dart';
@@ -318,7 +319,18 @@ class SkwasmMatrixColorFilter extends SkwasmColorFilter {
   @override
   void withRawColorFilter(ColorFilterHandleBorrow borrow) {
     withStackScope((scope) {
-      final rawColorFilter = colorFilterCreateMatrix(scope.convertDoublesToNative(matrix));
+      assert(matrix.length == 20);
+      final Pointer<Float> rawMatrix = scope.convertDoublesToNative(matrix);
+
+      /// Flutter documentation says the translation column of the color matrix
+      /// is specified in unnormalized 0..255 space. Skia expects the
+      /// translation values to be normalized to 0..1 space.
+      ///
+      /// See [https://api.flutter.dev/flutter/dart-ui/ColorFilter/ColorFilter.matrix.html].
+      for (final i in <int>[4, 9, 14, 19]) {
+        rawMatrix[i] /= 255.0;
+      }
+      final rawColorFilter = colorFilterCreateMatrix(rawMatrix);
       borrow(rawColorFilter);
       colorFilterDispose(rawColorFilter);
     });

engine/src/flutter/lib/web_ui/test/ui/filters_test.dart

@@ -146,59 +146,39 @@ Future<void> testMain() async {
 
   test('matrix color filter', () async {
     const ui.ColorFilter sepia = ui.ColorFilter.matrix(<double>[
-      0.393,
-      0.769,
-      0.189,
-      0,
-      0,
-      0.349,
-      0.686,
-      0.168,
-      0,
-      0,
-      0.272,
-      0.534,
-      0.131,
-      0,
-      0,
-      0,
-      0,
-      0,
-      1,
-      0,
+      0.393, 0.769, 0.189, 0, 0, // row
+      0.349, 0.686, 0.168, 0, 0, // row
+      0.272, 0.534, 0.131, 0, 0, // row
+      0, 0, 0, 1, 0, // row
     ]);
     await drawTestImageWithPaint(ui.Paint()..colorFilter = sepia);
     await matchGoldenFile('ui_filter_matrix_colorfilter.png', region: region);
     expect(sepia.toString(), startsWith('ColorFilter.matrix([0.393, 0.769, 0.189, '));
   });
 
+  test('matrix color filter with 0..255 translation values', () async {
+    const ui.ColorFilter sepia = ui.ColorFilter.matrix(<double>[
+      0.393, 0.769, 0.189, 0, 50.0, // row
+      0.349, 0.686, 0.168, 0, 50.0, // row
+      0.272, 0.534, 0.131, 0, 50.0, // row
+      0, 0, 0, 1, 0, // row
+    ]);
+    await drawTestImageWithPaint(ui.Paint()..colorFilter = sepia);
+    await matchGoldenFile('ui_filter_matrix_colorfilter_with_translation.png', region: region);
+    expect(sepia.toString(), startsWith('ColorFilter.matrix([0.393, 0.769, 0.189, '));
+  });
+
   test('invert colors', () async {
     await drawTestImageWithPaint(ui.Paint()..invertColors = true);
     await matchGoldenFile('ui_filter_invert_colors.png', region: region);
   });
 
   test('invert colors with color filter', () async {
     const ui.ColorFilter sepia = ui.ColorFilter.matrix(<double>[
-      0.393,
-      0.769,
-      0.189,
-      0,
-      0,
-      0.349,
-      0.686,
-      0.168,
-      0,
-      0,
-      0.272,
-      0.534,
-      0.131,
-      0,
-      0,
-      0,
-      0,
-      0,
-      1,
-      0,
+      0.393, 0.769, 0.189, 0, 0, // row
+      0.349, 0.686, 0.168, 0, 0, // row
+      0.272, 0.534, 0.131, 0, 0, // row
+      0, 0, 0, 1, 0, // row
     ]);
 
     await drawTestImageWithPaint(