Added initial support for Progressive decoding

This release adds initial support for progressive decoding. Normally progressive decoding requires a lot of RAM because the entire image must be kept in memory as each successive "scan" is decoded with more detailed DCT blocks. For this release, I added support for decoding the first scan which normally just contains the DC values. This allows me to offer a thumbnail sized (1/8th) decode without requiring more RAM. I can add full progressive support later, but it seems like a very rare MCU that would have the need (and the large spare RAM) to use it.

Shoutout to Tobias Butler (Tuneshine) for sponsoring this feature.