[CM] S7 wrapper around SDL

[chohag at jtan.com]

A project I am working on needs to use SDL and OpenGL (among others)
from S7 and so I am writing an S7 interface to SDL, which I have
included below.

The files can be loaded without any fuss into a fresh S7 context.
The compiler and linker settings are hardcoded to values valid on
OpenBSD at the bottom of the SCM files.

I'm sending it to this list aside from my main project, which is a
huge mess, to get feedback on my implementation before anything
gets too set in stone. What's good, what's bad, what could be done
better or just differently?

Any thoughts or suggestions are very welcome, don't hold back!

I have others, a matrix maths library, nuklear GUI and some of my
own modules, and I think I will need sqlite very soon, but SDL will
do for now and this will also inform how I get on with those.

Cheers,

Matthew

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