Shaders are no longer found under /shaders in the project; they can be
anywhere. In addition, paths for both (program) instructions in the
script and include directives in shader source code are relative to the
file which contains them.
* It doesn't work for function arguments at this point, but shouldn't be
too hard to implement that.
* Can't use it to its full potential mostly because I need to get rid of
the node-specific shit in the AST.
