Use references where NULL pointers aren't allowed.
Use const arguments where appropriate.
Apply a more conventional indent style.
Add a default ctor for ttexture.
Add a function that clears the window with a specified color.
Add a function to check if the handled pointer is NULL.
Make the global twindow object globally available.

Note: This is a quick an not really elegant change. I'll fix this as
soon as I'll have figured how CVideo and twindow should relate to
eachother.
Use SDL2's render API to clear the background and render the logo.

Only affects SDL2 builds.