Replace hardcoded frame number in plDTProgressMgr.

This was overlooked when the consolidation was done in 3027e0605c2.
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.
Merge pull request #370 from Deledrius/framenum_fix

Replace hardcoded frame number in plDTProgressMgr.