/// <summary>
/// remove old unused elements from cache
/// </summary>
public static void purge()
{
int cacheMinCount = Project.CACHE_MIN_UNUSED_COUNT;
//LibSys.StatusBar.Trace("IP: TerraserverCache:purge() before: " + ToString());
m_backdropCache.purge(cacheMinCount);
//LibSys.StatusBar.Trace("IP: TerraserverCache:purge() after: " + ToString());
}