Free base tilesheet image after processing it

This isn't a memory leak (not even Valgrind complains), because it gets
properly cleaned up in GraphicsResources::destroy(). Still, it's memory
that is just laying around not being used, and in the name of
deallocating things as soon as you no longer need them, we should
deallocate the base tilesheet images after we split all of them into
tiles.

This reduces the memory cost of all tilesheet images by half, since we
were essentially keeping around duplicates for nothing; this doesn't
really have much of an impact with conventional tilesheet sizes, since
they're usually small enough, but since 2.3 allowed for tilesheet images
of any size, this is a pretty big deal for really big tilesheet images.

It's okay to do this, even though they also get freed in
GraphicsResources::destroy(), because SDL_FreeSurface() does a NULL
check on the pointer passed to it, and we set the pointer to NULL after
freeing the surfaces.

desktop_version/src/Graphics.cpp

@@ -341,7 +341,10 @@ void Graphics::updatetitlecolours()
             \
             extra_code \
         } \
-    }
+    } \
+    \
+    SDL_FreeSurface(grphx.im_##tilesheet); \
+    grphx.im_##tilesheet = NULL;
 
 #define PROCESS_TILESHEET(tilesheet, tile_square, extra_code) \
     PROCESS_TILESHEET_RENAME(tilesheet, tilesheet, tile_square, extra_code)