[client] overlay: render splash fade from monotonic time

Move splash transitions onto the render thread and derive opacity from
elapsed monotonic time. Pace active animation at a display-compatible
minimum of 60 Hz while allowing incoming frames to satisfy that demand.

Reset the splash cleanly across reconnects and latch state changes with
the full-frame restoration decision.

client/include/interface/overlay.h

@@ -41,7 +41,7 @@ struct LG_OverlayOps
   /* final free */
   void (*free)(void * udata);
 
-  /* return true if realtime rendering is required when in jitRender mode
+  /* return true if realtime rendering is required
    * optional, if omitted assumes false */
   bool (*needs_render)(void * udata, bool interactive);