Boost field drawing framerate

Packets published without field overlays that happened to land
at the end of a telemetry batch would previously prevent any field
overlay from being shown from that batch. This doesn't cause
flickering, but it does reduce the framerate of field updates.
Now the most recent non-empty field overlay is kept in each batch.

Author: rbrott

DashboardCore/src/main/java/com/acmerobotics/dashboard/DashboardCore.java

@@ -66,12 +66,20 @@ public void run() {
                         pendingTelemetry.clear();
                     }
 
-                    // only the latest packet field overlay is used
+                    // only the latest packet non-empty field overlay is used
                     // this helps save bandwidth, especially for more complex overlays
-                    for (TelemetryPacket packet : telemetryToSend.subList(0,
-                        telemetryToSend.size() - 1)) {
-                        packet.field().clear();
-                        packet.fieldOverlay().clear();
+                    for (int i = telemetryToSend.size() - 1; i >= 0; i--) {
+                        TelemetryPacket packet = telemetryToSend.get(i);
+                        if (!packet.fieldOverlay().getOperations().isEmpty()) {
+                            for (int j = 0; j < i; j++) {
+                                TelemetryPacket packet2 = telemetryToSend.get(j);
+                                packet2.field().clear();
+                                packet2.fieldOverlay().clear();
+                            }
+                            break;
+                        } else {
+                            packet.field().clear();
+                        }
                     }
 
                     sendAll(new ReceiveTelemetry(telemetryToSend));