Demo doesn't include code on how to display images without OpenGL.

[malespiaut]

Hello,

I was reading the demo code for the sdl_renderer, but when I looked for examples on how to display a nk_image, I've only founc examples that requires OpenGL.

The demo code should, in my humble opinion, have example that covers displaying images without the use of OpenGL.

Best regards.

[RobLoach]

Wiki has image examples for SDL https://github.com/Immediate-Mode-UI/Nuklear/wiki/SDL

Would like a common image example, perhaps handled via this pr #466