Instead of compiling all variants of all demos, load the best backend at
runtime. Also split off the nuklear implementation into a separate
library to avoid excessively recompiling it, something that should have
been done ages ago.

This is technically a functionality downgrade, since it doesn't allow
users to choose which backend to use (Vulkan or OpenGL), instead always
picking the "best" backend. But that's mostly interesting for
developers, not users, I think.

I'll probably implement some mechanism for making that selection at
runtime, when I actually need it.

As an aside, I also renamed 'image' to 'sdlimage' since I think the name
fits better - the only reason I didn't in the past was to avoid
confusion with the suffix.