Preloaded art setting - reduce loading time

[teffy]

If you click "preload art" you waste a lot of bandwith and loading time getting all these card art with separate "GET- Requests" - all card arts are requested as single files.

Using sprites like http://etg.dek.im/assets/atlas.png could help.

[serprex]

Loading time of preload could also be improved by requesting more than 1 file at a time (better would be 3-4)

Ideally we'd be using http2