Support for #include in PShader (Enhancement)

Created by: patriciogonzalezvivo

Although #include routines are defined by Khronos as an extension they are left for the environment/framework running the GL driver to implement. The reason for that is the required access to file system. Implementing #include routines are fairly straight forward with a couple of gotchas to avoid, like: relative paths and infinite recursions.
Most serious CG engines support them because their advantages on modularity, reusability and clearer code.
For Processing this also could mean support for LYGIA a rich shader library pack with useful resources for creative coders and artist.
I made a simple implementations of a #include parser in this repository together with some examples on how it can be use.

void loadSource(String current_folder, String filename, ArrayList<String> source) {
  println("search for", filename, "in", current_folder);
  File file = new File(current_folder,filename);
  String url = file.getPath().substring(1);
  println("open", url);
  String[] lines = loadStrings(url); 
  
  for (int i = 0; i < lines.length; i++) {
    String line = lines[i];
    String line_trim = line.trim();
    if (line_trim.startsWith("#include")){
      String include_file = line_trim.substring("#include".length()).replace("\"", "").trim();
      File f = new File(current_folder,include_file);
      loadSource(f.getParent(), f.getName(), source);
    } else {
      source.add(line + System.getProperty("line.separator") );
    }
  }
}

Adding this feature to createShader() to parse both vert and frag shaders shouldn't be hard and potentially bring numerous advantages.

cc @codeanticode @SableRaf @cacheflowe

Start by locating createShader() and the code paths that load vertex and fragment PShaders. Read the linked lygia_p5_examples include parser for its handling of relative paths and recursion, then identify existing shader-loading tests or examples. Done means both shader types can expand #include files safely, including nested relative includes, without infinite recursion.