This function takes a surface and copies it to a new surface of the +pixel format and colors of the video framebuffer plus an alpha channel, +suitable for fast blitting onto the display surface. It calls +SDL_ConvertSurface
If you want to take advantage of hardware colorkey or alpha blit +acceleration, you should set the colorkey and alpha value before +calling this function.
This function can be used to convert a colourkey to an alpha channel, +if the SDL_SRCCOLORKEY flag is set on the surface. +The generated surface will then be transparent (alpha=0) where the +pixels match the colourkey, and opaque (alpha=255) elsewhere.