Support for >8 bit depth output

[ChiliJohnson]

I see that PNG output support was just announced at #102 (comment) with 16-bit PNG support coming down the pipeline which is great! I didn't see any existing issue tracking this and I'm very interested in this feature so I'm making one here.

Aside from 16-bit depth support for PNG, it could also be nice to support 16-bit JPEG2000 as an output format too, so I'm setting up this issue to track any general >8 bit depth support in iipsrv