Strife: improve TrueColor crossfading effect

[JNechaevsky]

That's it, as simple as it gets. If feels slightly longer than original effect, presumably by just a few tics (yeah, inner timer is a thing), but looks absolutely fine and smooth.

TODOs:

• Not sure why wipe_scr wants to be I_VideoBuffer. Should it be Z_Malloc'ed like in paletted render, then effect will be notably faster in speed and less epic. Same applies to drawing solid background while crossfading effect, but it shouldn't be that CPU-expensive, as the effect itself is gametic based.
• Memory leak is still happening. That's odd, since all pixel_t variables are seems to be freed. Fixed!

[JNechaevsky]

@fabiangreffrath, your word, please. 🙂