[client] renderers: treat -1 as full damage and 0 as no overlay

This makes everything consistent.

client/include/app.h

@@ -85,8 +85,8 @@ void app_registerOverlay(const struct LG_OverlayOps * ops, void * params);
 /**
  * render the overlay
  * returns:
- *   -1 for no overlay
- *    0 for full output damage
+ *   -1 for full output damage
+ *    0 for no overlay
  *   >0 number of rects written into rects
  */
 int app_renderOverlay(struct Rect * rects, int maxRects);