Fix 'ImageFilter.shader can be applied to canvas operations' test (flutter#180929)

The test draws a red pixel onto a canvas with an ImageFilter that swaps
colors channels with
`frag_color = texture(u_texture, FlutterFragCoord().xy / u_size).bgra;`.

This PR fixes two things:
- When reading the output color, we can't construct a `Color` directly
from the byte data. The byte data is RGBA. But the `Color` int
constructor takes ARGB. So this was incorrectly reading the output
color. Instead, read the RGBA components individually and use them to
construct a color with `Color.fromARGB`.
- The input color red, when given to the color-swapping `bgra` shader,
outputs blue (0xFF0000FF), not green (0xFF00FF00).

Fixes flutter#180927

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Author: b-luk

engine/src/flutter/testing/dart/fragment_shader_test.dart

@@ -543,11 +543,6 @@ void main() async {
   });
 
   _runImpellerTest('ImageFilter.shader can be applied to canvas operations', () async {
-    // TODO(gaaclarke): This test was disabled for a long time and has been
-    // atrophied. Fix it or remove it.
-    print('Atrophied test is disabled.');
-    return;
-    // ignore: dead_code
     final FragmentProgram program = await FragmentProgram.fromAsset('filter_shader.frag.iplr');
     final FragmentShader shader = program.fragmentShader();
     final recorder = PictureRecorder();
@@ -558,10 +553,22 @@ void main() async {
         ..imageFilter = ImageFilter.shader(shader),
     );
     final Image image = await recorder.endRecording().toImage(1, 1);
-    final ByteData data = (await image.toByteData())!;
-    final color = Color(data.buffer.asUint32List()[0]);
 
-    expect(color, const Color(0xFF00FF00));
+    // Image's byte data consists of color values for each pixel in RGBA format. The image is 1
+    // pixel, so the byte data is expected to be 4 bytes.
+    final ByteData data = (await image.toByteData())!;
+    expect(data.lengthInBytes, 4);
+
+    final Uint8List colorComponentsRGBA = data.buffer.asUint8List();
+    final color = Color.fromARGB(
+      colorComponentsRGBA[3],
+      colorComponentsRGBA[0],
+      colorComponentsRGBA[1],
+      colorComponentsRGBA[2],
+    );
+    // filter_shader.frag swaps red and blue color channels. The drawn color is red, so the expected
+    // result color is blue.
+    expect(color, const Color(0xFF0000FF));
   });
 
   // For an explaination of the problem see https://github.com/flutter/flutter/issues/163302 .