Implement new PCX image loader and add tests

[a740g]

This PR fixes the .pcx image loader issue reported here.

It has the following changes:

• New .pcx image loader
• Adds tests for most PCX sub-format: 1bpp, 2bpp, 4bpp, 8bpp and 24bpp
• Moves some inline colors functions out of qbx.cpp to image.h
• Updates library files to latest versions: stb_image, nanosvg