redesigned drawing of loading plaque, it's now a separate refresh function which...

redesigned drawing of loading plaque, it's now a separate refresh function which draws over the previous frame, rather than invoking a new render, which should fix crashes with level changes with rtlights on (trying to render an rtlight while the map the rtlight was for has already been unloaded tends to cause a crash)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@4796 d7cf8633-e32d-0410-b094-e92efae38249