TINSEL: Fix DW2 assertion when quitting during cutscene

Call _bmv->FinishBMV() before setting g_pCurBgnd to NULL. Otherwise,
there will be an assertion if quitting during a cutscene while a
subtitle is being shown. (At least I think that's the condition.)

(Also, it seems to be a good idea to call it before EndScene(),
because even though there was no assertion, I got a lot of warnings
when I did it after.)