1.  Use SDL_X11_HAVE_XRENDER to check for RENDER at runtime.
2.  Added lots of comments.
3.  Added checks and lots of calls to SDL_SetError().
4.  Fixed X11_CreateTexture() so that the pixmap and image created
    are for the format specified by the user and not the window
    format. This is only for the RENDER case.
5.  The above change required that functions to convert SDL
    pixel format enums to Visuals and XRenderPictFormats be added.
6.  Fixed lots of 'style' issues.