Support an alpha channel.
Improve the PNG interface.
Global parameters to control encoder settings:
- Quantization level: quant 0
- Quantization level: quants 1-63
  - Constant term
  - Linear term
- Number of quants to keep, per tile.
- Whether to use wide or packed representation for the thumbnail block
Interleave layers instead of having them consecutively.
Maybe have a second instance of the global parameters block for the alpha channel.
Actually support terminating early for "progressive" loads