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hqx is a fast, high-quality magnification filter designed for pixel art.
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These changes has been made to suit the needs of Brum Brum Rally:

 * The RGBtoYUV array is dynamically allocated in hqxInit() to avoid
   wasting memory if the hqx filter is not used.

 * The scaling functions are able to handle parts of an image to avoid
   having to redraw everything unless needed.

 * White color is handled like any other color (Not sure if this was a 
   bug or by design in the original code).
