Roland DJ-505: Add rudimentary support for the jog wheel LED indicators

Missing support was discovered by Daniél on Zulip:
https://mixxx.zulipchat.com/#narrow/stream/109695-_support/topic/for.20mevsme/near/187302854

res/controllers/Roland_DJ-505-scripts.js

@@ -438,6 +438,44 @@ DJ505.Deck = function(deckNumbers, offset) {
         }
     };
 
+    /* Platter Spin LED Indicator on Jog Wheels
+     *
+     * The Controller features a LED indicator that imitates a spinning
+     * platter when the deck is playing and also shows to position inside a
+     * bar.
+     *
+     * LED indicator values:
+     * - 0x00 - 0x1F: Beat 0 (downbeat) to 1
+     * - 0x20 - 0x3F: Beat 1 to 2
+     * - 0x40 - 0x5F: Beat 2 to 3
+     * - 0x60 - 0x7F: Beat 3 (upbeat) to 0 (next downbeat)
+     *
+     * TODO: Add proper bar support for the LED indicators
+     *
+     * Mixx currently does not support bar detection, so we don't know which
+     * of the 4 beats in a we are on. This has been worked around by counting
+     * beats manually, but this is error prone and does not support moving
+     * backwards in a track, has problems with loops, does not detect hotcue
+     * jumps and does not indicate the downbeat (obviously).
+     *
+     * See Launchpad issue: https://bugs.launchpad.net/mixxx/+bug/419155
+     */
+    this.beatIndex = 0;
+    this.lastBeatDistance = 0;
+    engine.makeConnection(this.currentDeck, "beat_distance", function(value) {
+        // Check if we're already in front of the next beat.
+        if (value < this.lastBeatDistance) {
+            this.beatIndex = (this.beatIndex + 1) % 4;
+        }
+        this.lastBeatDistance = value;
+
+        // Since deck indices start with 1, we use 0xAF + deck for the status
+        // byte, so that we 0xB0 for the first deck.
+        var status = 0xAF + script.deckFromGroup(this.currentDeck);
+
+        // Send a value between 0x00 and 0x7F to set jog wheel LED indicator
+        midi.sendShortMsg(status, 0x06, Math.round(0x1f * value + 0x20 * this.beatIndex));
+    });
 
     // ========================== LOOP SECTION ==============================