Set SDL_GL_ACCELERATED_VISUAL to the actual value after OpenGL context was created

[cher-nov]

So that it could be retrieved by calling SDL_GL_GetAttribute() to determine whether the newly created context is accelerated.

There's also already a FIXME related to that:

SDL/src/video/SDL_video.c

Lines 4850 to 4855 in 4b429b9

	case SDL_GL_ACCELERATED_VISUAL:
	{
	// FIXME: How do we get this information?
	*value = (_this->gl_config.accelerated != 0);
	return true;
	}