Replace hardcoded frame number in plDTProgressMgr.

This was overlooked when the consolidation was done in 3027e0605c.
With this fix, the number of frames to be loaded is entirely defined
in a single place.  This also prepares the code for a future commit
to remove the necessity of the compile-time definition entirely.

Sources/Plasma/PubUtilLib/plPipeline/plDTProgressMgr.cpp

@@ -157,7 +157,7 @@ void    plDTProgressMgr::Draw( plPipeline *p )
         if ((currentMs - fLastDraw) > 30)
         {
             fCurrentImage++;
-            if (fCurrentImage >= 18)
+            if (fCurrentImage >= plProgressMgr::NumLoadingFrames())
                 fCurrentImage = 0;
 
             fLastDraw = currentMs;