retry mapmodel downloads

Mapmodels which were not found once are put on a list and not tried to
be loaded again. With this commit, the list is cleared during a map load
if autodownload is enabled. This causes all missing models to be
downloaded.

Fixes issues with manually aborted autodownloads.

source/src/rendermodel.cpp

@@ -305,6 +305,12 @@ void cleanupmodels()
     enumerate(mdllookup, model *, m, m->cleanup());
 }
 
+void resetmdlnotfound()
+{
+    enumeratek(mdlnotfound, const char *, m, delstring(m));
+    mdlnotfound.clear();
+}
+
 void getmapmodelattributes(char *name, char *attr)
 {
     const char *res = NULL;