disable mouse-over-image when no lua callback is registered

this should improve overall responsiveness quite a bit

this is a horrible hack which is OK in master but shouldn't
be ported to master. A whole rewrite of that area should
be done to fix this for real

src/lua/events.c

@@ -211,6 +211,9 @@ static int lua_register_event(lua_State *L)
     lua_pop(L, 2);
     return luaL_error(L, "unknown event type : %s\n", evt_name);
   }
+  if(!strcmp(evt_name,"mouse-over-image-changed")) {
+    darktable.lua_state.mouse_event_registered = true;
+  }
   lua_getfield(L, -1, "on_register");
   lua_getfield(L, -2, "data");
   for(int i = 1; i <= nparams; i++) lua_pushvalue(L, i);

src/lua/gui.c

@@ -216,8 +216,10 @@ static int lua_job_valid(lua_State *L)
   }
 }
 
+
 static void on_mouse_over_image_changed(gpointer instance, gpointer user_data)
 {
+  if(!darktable.lua_state.mouse_event_registered) return;
   int imgid = dt_control_get_mouse_over_id();
   if(imgid != -1) {
   dt_lua_do_chunk_async(dt_lua_event_trigger_wrapper,

src/lua/init.c

@@ -80,6 +80,7 @@ void dt_lua_init_early(lua_State *L)
   }
   darktable.lua_state.state = L;
   darktable.lua_state.ending = false;
+  darktable.lua_state.mouse_event_registered = false;
   darktable.lua_state.pending_threads = 0;
   dt_lua_init_lock(); // lock is initialized in the locked state
   luaL_openlibs(darktable.lua_state.state);

src/lua/lua.h

@@ -69,6 +69,10 @@ typedef struct
   dt_pthread_mutex_t mutex;
   int pending_threads ;
   bool ending;
+// HORRIBLE HACK, STABLE BRANCH ONLY
+// allow the mouse event to be skipped very early
+// if no callback for it was registered
+  bool mouse_event_registered;
 
 } dt_lua_state_t;