Method invert

src/Engine/Surface.cpp:451–471

* Inverts all the colors in the surface according to a middle point. * Used for effects like shifting a button between pressed and unpressed. * @param mid Middle point. */

 * @param mid Middle point.
 */
void Surface::invert(Uint8 mid)
{
	// Lock the surface
	lock();

	for (int x = 0, y = 0; x < getWidth() && y < getHeight();)
	{
		Uint8 pixel = getPixel(x, y);
		if (pixel > 0)
		{
			setPixelIterative(&x, &y, pixel + 2 * ((int)mid - (int)pixel));
		}
		else
		{
			setPixelIterative(&x, &y, 0);
		}
	}

	// Unlock the surface
	unlock();
}

/**
 * Runs any code the surface needs to keep updating every