vwebp: Clear previous frame when a key triggers a redraw

otherwise, transparent areas were accumulating.

Change-Id: I066a96a2bcf0cac750b3df0c02229542b1ed3473
(cherry picked from commit c0a27fd2af)

examples/vwebp.c

@@ -242,6 +242,9 @@ static void HandleKey(unsigned char key, int pos_x, int pos_y) {
     }
   } else if (key == 'i') {
     kParams.print_info = 1 - kParams.print_info;
+    // TODO(skal): handle refresh of animation's last-frame too. It's quite
+    // more involved though (need to save the previous frame).
+    if (!kParams.has_animation) ClearPreviousFrame();
     glutPostRedisplay();
   }
 }