GraphicsEngine now stores a Bitmap pointer for the screen instead of
a SDL_Surface. Adjusted calling code accordingly.
With this change, removing GraphicsEngine::StrokeRect was possible.
Changed colors for polygons, to better debug the FillPolygon code.
Add some output while building the blit mask.
Don't save the blit mask to file.
Fix FillPolygon. The node list needs to be an unsigned variable (we use uint32).
First multiply, then divide, to avoid losing precision.