Base+LibCards: Add basic set of playing card backs

[lemtom]

As discussed in the Discord, this is a basic set of playing card backs. One of them replaces an existing yak silhouette card back, the other two existing card cards have not been touched. I have run optipng on my files as suggested in the contributing guidelines.

The set consists of two simple patterned card backs, two classic animal-themed card backs inspired by old-timey playing cards, and two modern animal-themed card backs. The intent is to offer some amount of variety without being too excessive.

The two classic card backs include derivative work from public domain artwork (namely a yak and ladybug illustration). Everything else is entirely original. I've generated these from SVG files, though I'm not sure if / where those can be stored.

I release this work into the Public Domain using the Creative Commons 0 license.