Made JPEG reading completely independent from Lepton encoding

[mcroomp]

As part of the making the JPEG code useful outside of Lepton encoding, this change refactors the JPEG read code so that there is a single function "read_jpeg_file" that contains all the logic necessary to collect the information needed to reconstruct the JPEG image.

The write side still needs a bit of work to consolidate into a single API

[Melirius]

I suggest as a part of this nice refactoring to rename structure JPegHeader into JpegHeader, as well as enums JPegType and JPegDecodeStatus into JpegType and JpegDecodeStatus. I wonder why they are named like this :) JPEG is an acronym, so either all capitals, or consistent like this looks more logical to me.

[mcroomp]

I suggest as a part of this nice refactoring to rename structure JPegHeader into JpegHeader, as well as enums JPegType and JPegDecodeStatus into JpegType and JpegDecodeStatus. I wonder why they are named like this :) JPEG is an acronym, so either all capitals, or consistent like this looks more logical to me.

No idea about the original capitalization... I changed it to Jpeg