Fix regression with background of level minimaps not being black

I had misread this line in #629 and thought that it was just clearing
the entire surface, when really it was filling the surface with opaque
black. ClearSurface() would instead make it transparent, which would
mean when it got drawn, it would get drawn against blue, and not black.
Whoops.

desktop_version/src/editor.cpp

@@ -2152,7 +2152,7 @@ void editorclass::generatecustomminimap(void)
         map.custommmysize=180-(map.custommmyoff*2);
     }
 
-    ClearSurface(graphics.images[12]);
+    FillRect(graphics.images[12], graphics.getRGB(0, 0, 0));
 
     int tm=0;
     int temp=0;