Decouple image compression from image loading

[WillyJL]

• images should load in whatever format is available, with preference for selected compression mode (as is currently)
• compression shouldn't be a prerequisite for loading the image
• run a thread in background that periodically checks what images arent compressed and compresses them
  • maybe not periodically, but rather run once at startup then keep track of when image files are added/replaced
  • thread will unload a game's image when compression for it is completed
  • we can probably skip compression for preview images, they don't need instantaneous loading, even if its a few seconds it should be fine, and the tradeoff to much higher space usage isnt worth it
• need to figure out a synchronization method between api/main thread wanting to replace the image, and compression thread compressing the old image
  • maybe a mutex is sufficient, or maybe it should be possible for replacing image to interrupt the compression of the older image
• maybe imagehelper can keep the prior frames right up until the new ones are ready, to avoid flicker?
  • needs to still discard the old ones if new ones fail loading, so error is shown