Updated base node class to support several outputs.
Now bricks support 2 outputs : greyscale depth map, and (random) brick color.
Now Voronoi supports 3 outputs : center distance, edge distance and cell color.
Updated bricks example to use brick color output.
Added Image node that can be used to import textures
Renamed all get_shader_code methods to _get_shader code so a caching system can be integrated in node_base (and we can get rid of get_source_rgb and get_source_f functions)
