KYRA: Rewrite the VQA decoder using the VideoDecoder classes

This is my attempt at rewriting the Kyrandia 3 VQA player using ScummVM's video decoder class. It also cleans up/simplifies a few things the old code did.

I do have somewhat mixed feelings about the changes I had to make to the functions in screen.cpp, since they assume that the width and the pitch are equal (not necessarily true for a Surface), but other than that I'm fairly happy with how it turned out. I have played through all the cutscene files (though only by using them as intros from the main menu, not from within the game itself), and I couldn't see any obvious problems.