[web] Add basic color per vertex drawVertices API support (#13066)

* Implement basic drawVertices for BlendMode.srcOver and hairline rendering
* Implement maxDiffRate parameter for screenshot tests

Author: ferhatb

lib/web_ui/dev/goldens_lock.yaml

@@ -1,2 +1,2 @@
 repository: https://github.com/flutter/goldens.git
-revision: 7efcec3e8b0bbb6748a992b23a0a89300aa323c7
+revision: 686dd320f6cce6da9a7a43e3ec9c0147f39eb19d

lib/web_ui/dev/test_platform.dart

@@ -146,12 +146,13 @@ class BrowserPlatform extends PlatformPlugin {
     final Map<String, dynamic> requestData = json.decode(payload);
     final String filename = requestData['filename'];
     final bool write = requestData['write'];
+    final double maxDiffRate = requestData['maxdiffrate'];
     final Map<String, dynamic> region = requestData['region'];
-    final String result = await _diffScreenshot(filename, write, region);
+    final String result = await _diffScreenshot(filename, write, maxDiffRate ?? _kMaxDiffRateFailure, region);
     return shelf.Response.ok(json.encode(result));
   }
 
-  Future<String> _diffScreenshot(String filename, bool write, [ Map<String, dynamic> region ]) async {
+  Future<String> _diffScreenshot(String filename, bool write, double maxDiffRateFailure, [ Map<String, dynamic> region ]) async {
     if (doUpdateScreenshotGoldens) {
       write = true;
     }
@@ -282,7 +283,7 @@ Golden file $filename did not match the image generated by the test.
 
       final StringBuffer message = StringBuffer();
       message.writeln('Golden file $filename did not match the image generated by the test.');
-      message.writeln(getPrintableDiffFilesInfo(diff.rate, _kMaxDiffRateFailure));
+      message.writeln(getPrintableDiffFilesInfo(diff.rate, maxDiffRateFailure));
       message.writeln('You can view the test report in your browser by opening:');
 
       // Cirrus cannot serve HTML pages generated by build jobs, so we
@@ -314,7 +315,7 @@ Golden file $filename did not match the image generated by the test.
       message.writeln('Golden file: ${expectedFile.path}');
       message.writeln('Actual file: ${actualFile.path}');
 
-      if (diff.rate < _kMaxDiffRateFailure) {
+      if (diff.rate < maxDiffRateFailure) {
         // Issue a warning but do not fail the test.
         print('WARNING:');
         print(message);