Initialize bcol and bcol2 in Graphics::drawbackground()

While compiling in release mode, GCC warns about these two potentially
being used uninitialized further down. The only way this could happen is
if the case-switches below didn't match up with a case, which would
require the game to be in an invalid state (and have invalid values for
rcol and spcol), but it's better to be safe than sorry.

desktop_version/src/Graphics.cpp

@@ -1932,7 +1932,7 @@ void Graphics::drawbackground( int t )
         break;
     case 2:
     {
-        int bcol, bcol2;
+        int bcol = 0, bcol2 = 0;
 
             //Lab
             switch(rcol)