Force full screen refresh with a rounder function

When the screen switches, the full screen needs to be refreshed for the
hardware scrolling to work. This was enforced with backgroundLabels, but
is simpler to do with a rounder function.

src/displayapp/screens/WatchFaceDigital.h

@@ -59,7 +59,6 @@ namespace Pinetime {
         lv_obj_t* label_time;
         lv_obj_t* label_time_ampm;
         lv_obj_t* label_date;
-        lv_obj_t* backgroundLabel;
         lv_obj_t* bleIcon;
         lv_obj_t* batteryPlug;
         lv_obj_t* heartbeatIcon;