regroup opengl version and window creation and event polling in the
viewport file.
To further clean up the files layout, move input.lisp to engine folder
gl-utils is now used for some simple macros
Resources are loaded as binary or lisp files. A streamed resource is
also planned but not yet implemented.
Resources are shared and loading happens asynchronically,
powered by promises.
Resources are used through proxies that shares a weak pointer.
When a proxy is gc'd, a finalizer is triggered using trivial-garbage to
release the resource.
