new CustomBackground callback

[wkjarosz]

fixes #70

I wasn't sure whether the markdown documentation is autogenerated from the code comments, but, just in case, I included the documentation for the new callback both in the code and the markdown documentation.

Also, I moved the Impl_Frame_3D_ClearColor() (or CustomBackground) call to be just before RenderGui in CreateFramesAndRender(). Otherwise the Gui is delayed by 1 frame compared to the background.

[pthom]

Hello,

Thanks a lot for your contribution! It was perfect, so I merged it.

I wasn't sure whether the markdown documentation is autogenerated from the code comments

Yes, it is. See tools/doc/process_md_docs.py. It is called automatically if you invoke cmake with -DHELLOIMGUI_BUILD_DOCS=ON

Since it was worthy of a demo + documentation I added a demo: see a7afb9e

Thanks again!