Actually support terminating early for "progressive" loads

Done:
Support an alpha channel.
Improve the PNG interface.
Interleave layers instead of having them consecutively. (Bad idea, breaks RLE. Skipping)
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
Maybe have a second instance of the global parameters block for the alpha channel.