Ax OverlaySurfaceKeyed(), set proper foregroundBuffer blend mode

So, earlier in the development of 2.0, Simon Roth (I presume)
encountered a problem: Oh no, all my backgrounds aren't appearing! And
this is because my foregroundBuffer, which contains all the drawn tiles,
is drawing complete black over it!

So he had a solution that seems ingenius, but is actually really really
hacky and super 100% NOT the proper solution. Just, take the
foregroundBuffer, iterate over each pixel, and DON'T draw any pixel
that's 0xDEADBEEF. 0xDEADBEEF is a special signal meaning "don't draw
this pixel". It is called a 'key'.

Unfortunately, this causes a bug where translucent pixels on tiles
(pixels between 0% and 100% opacity) didn't get drawn correctly. They
would be drawn against this weird blue color.

Now, in #103, I came across this weird constant and decided "hey, this
looks awfully like that weird blue color I came across, maybe if I set
it to 0x00000000, i.e. complete and transparent black, the issue will be
fixed". And it DID appear to be fixed. However, I didn't look too
closely, nor did I test it that much, and all that ended up doing was
drawing the pixels against black, which more subtly disguised the
problem with translucent pixels.

So, after some investigation, I noticed that BlitSurfaceColoured() was
drawing translucent pixels just fine. And I thought at the time that
there was something wrong with BlitSurfaceStandard(), or something.
Further along later I realized that all drawn tiles were passing through
this weird OverlaySurfaceKeyed() function. And removing it in favor of a
straight SDL_BlitSurface() produced the bug I mentioned above: Oh no,
all the backgrounds don't show up, because my foregroundBuffer is
drawing pure black over them!

Well... just... set the proper blend mode for foregroundBuffer. It
should be SDL_BLENDMODE_BLEND instead of SDL_BLENDMODE_NONE.

Then you don't have to worry about your transparency at all. If you did
it right, you won't have to resort this hacky color-keying business.

*sigh*

desktop_version/src/GraphicsUtil.h

@@ -39,8 +39,6 @@ void FillRect( SDL_Surface* surface, SDL_Rect& rect, const int r, int g, int b )
 
 void FillRect( SDL_Surface* surface, SDL_Rect rect, int rgba );
 
-void OverlaySurfaceKeyed(SDL_Surface* _src, SDL_Surface* _dest, Uint32 _key);
-
 void ScrollSurface(SDL_Surface* _src, int pX, int py);
 
 SDL_Surface * FlipSurfaceHorizontal(SDL_Surface* _src);