[PATCH] Implement software DXT1 compression

[alberto at mattea.info]

Hi Stefan. I'm not really into opengl internals, so I think the best
solution for now is to implement 1), and give complete support when the
blitting code will be in a better shape. Would this be okay?

    if (!context->valid)
    {
        context_release(context);
        FIXME("No gl context, cannot perform conversion\n");
        return;
    }

Thanks