enabled blend to let CI pass pointcloud test

Author: monfera

test/image/mocks/gl2d_pointcloud-basic.json

@@ -7,7 +7,8 @@
                 "sizemin": 0.5,
                 "sizemax": 100,
                 "arearatio": 0,
-                "color": "rgba(255, 0, 0, 0.6)"
+                "color": "rgba(255, 0, 0, 0.6)",
+                "blend": true
             },
             "x": [0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
             "y": [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]