Use screen dimensions on RPI, cache large images, add ini config support

[JarvyJ]

These are the last couple of things needed to get this going.

• [x] Use fbset to get the RPI's screen resolution and use that when initializing raylib
  • images should always display as the correct aspect ratio for the screen
• [x] Cache images larger than the screen size
  • big images take longer to load, so might as well make image loading quick
• [x] Use an env var to set caching directory
• [x] Use an ini file for slideshow settings

This PR will likely not cover cache invalidation. I could take care of that in the API or have users delete manually if needed.

I probably should've split this into multiple PRs.