do not clear the entire frame when paining the playground

ttrs.py

@@ -169,9 +169,9 @@ class Playground():
         return True
 
     def paint(self, frame, xpos, ypos):
-        for x in range(frame.dimension.width):
-            for y in range(frame.dimension.height):
-                frame.setPixel(x, y, (0, 0, 0))
+        for x in range(self.width):
+            for y in range(self.height):
+                frame.setPixel(xpos + x, ypos + y, (0, 0, 0))
         for b in set.union(self.blocks, *[ m.blocks for m in self.minos ]):
             frame.setPixel(xpos + int(b.pos.x), ypos + int(b.pos.y), b.color)