Extend CGI test image set

[nicola-sorace]

A more comprehensive set of test images is required. First step is to generate images with chess pieces on them (this is already setup, the script just needs cleaning up). It's probably a good idea to have the image labels follow the exact structure used in ChessCog.

A full list of variations that could be useful (can keep adding ideas):

• Empty chessboards
• Chessboards with semi-random piece distributions
• More varied lighting and camera positions
• Different materials / textures
• Different chess piece sets
• Background clutter and other imperfections

These should be structured into separate sub-folders, and automatically generated by script(s). I'm thinking of having only a handful of examples for each uploaded to Git, with the ability to easily generate more on-demand for a given category.