instrument: one VELOCITY deck for all three velocity curves, bipolar and off by default for pitch and filter

Payload v12 appends the new velocity->pitch curve and folds the retired filter velAmount into its now-bipolar curve, so pre-v12 projects reopen sounding identical. Preview button takes a drawn play triangle.

src/shell/instrument/editor_paint.cpp

@@ -70,7 +70,7 @@ void ReaSamplerEditor::paintSample(LICE_IBitmap* bmp, int w, int h) {
     paintDeck(bmp, fl);
 
     // The curve popup: a centered sheet over the whole face, drawn last.
-    if (curvePopupOpen_) paintCurvePopup(bmp, w, h);
+    if (curvePopup_ != CurveTarget::kNone) paintCurvePopup(bmp, w, h);
 
     // The piano strip's note-name chip overhangs its band, so it goes on top of everything.
     paintChromeTooltip(bmp, fl, w, h);