[client] overlay: let the overlay know if it's in interactive mode

This also removes the need for the flags member as the overlay can just
opt to not render if it's not in interactive mode.

client/include/interface/overlay.h

@@ -26,12 +26,6 @@
 
 #include "common/types.h"
 
-enum LG_OverlayFlags
-{
-  /* requires mouse interaction */
-  LG_OVERLAY_INTERACTIVE = (1U << 1U),
-};
-
 struct LG_OverlayOps
 {
   /* internal name of the overlay for debugging */
@@ -43,9 +37,6 @@ struct LG_OverlayOps
   /* final free */
   void (*free)(void * udata);
 
-  /* general state flags, may be changed at any time */
-  enum LG_OverlayFlags flags;
-
   /* perform the actual drawing/rendering
    *
    * `interactive` is true if the application is currently in overlay interaction