surface must be a SDL_Surface when calling BitmapFont.render

[namelivia]

First of all thanks to the maintainers for the effort.
I believe I've found a bug, I'm testing BitmapFont and I've been able to render text doing this:

font = sdl2.ext.BitmapFont(image, (size, size), mapping)
font.render_on(image, "TEXT", (size * len("TEXT"), 0))

so, I create the bitmap font and I'm able to render text using render_on.

However, I would like to create a new Sprite for my text, and by reading the documentation I think the way to do it would be just simply doing:

font = sdl2.ext.BitmapFont(image, (size, size), mapping)
font.render("TEXT")

But I get an exception stating surface must be a SDL_Surface. Is it a bug or am I misunderstanding anything?

I've done some debugging and tracking the origin of the error on these lines.

 sf = surface.SDL_CreateRGBSurface(0, tw, th, bpp, 0, 0, 0, 0)
        if not sf:
            raise SDLError()
        imgsurface = SoftwareSprite(sf, False)

Apparently even tho sf is an instance of: sdl2.surface.LP_SDL_Surface it seems not to be a valid first parameter for SoftwareSprite.

Also based mainly on this answer on stack overflow, it seems .contents is missing like:

 sf = surface.SDL_CreateRGBSurface(0, tw, th, bpp, 0, 0, 0, 0)
        if not sf:
            raise SDLError()
        imgsurface = SoftwareSprite(sf.contents, False)

I've tested this on a fork I've made and by doing so I've been able to get no exception and a SoftwareSprite instance back from the render method so I've opened a PR.