Makefile.am is too ugly, but I don't have a better solution right now.

- fused Makefile.am and src/Makefile.am
- adding back windows cross-compilation
- improved README.md a lot
- added configure option to enable debug, which greatly simplifies
  development when writing shaders
- removed dependency on LunarG's Vulkan SDK by having dependency only on
  Vulkan headers, library and slangc
- removed print C++ header because I have an old MinGW which doesn't
  support it. It will be restored in the future
- make every assets path relative to be able to build anywhere and run
  elsewhere
- moved stb_image.h inside source directory instead of making it a
  dependency, which is more aligned with stb philosophy
- moved assets inside source code to make it easy to compile and run the
  engine
- improved configure.ac by checking Vulkan headers version directly in
  the configure script instead of within C code, which allows us to also
  do theses checks when cross-compiling