Add some masks and set a higher threshold for comparing images

tests/src/python/test_qgsserver.py

@@ -180,7 +180,7 @@ class QgsServerTestBase(unittest.TestCase):
             control.setSizeTolerance(max_size_diff.width(), max_size_diff.height())
         return control.compareImages(control_image, max_diff), control.report()
 
-    def _img_diff_error(self, response, headers, image, max_diff=10, max_size_diff=QSize()):
+    def _img_diff_error(self, response, headers, image, max_diff=100, max_size_diff=QSize()):
         self.assertEqual(
             headers.get("Content-Type"), "image/png",
             "Content type is wrong: %s\n%s" % (headers.get("Content-Type"), response))