Dual purpose and implicit state in PngData

[kornelski]

PngData has both idat_data and raw_data, which are related, but used in different contexts and handled as independent fields.

This is problematic, because:

• It's possible to modify raw_data without invalidating idat_data. Currently the code has to keep track of that "manually" through a flag passed around. That feels fragile.

• Copying of PngData copies both of them, even when it's just for trials/reductions which are going to use only one of them.

• Alpha filter trials get filtered_image version of the idat that needs to be recompressed better later, but there's no way to store the filtered version, so later recompression does filtering again (that's very minor though).

I'm not entirely sure how this should be handled, but there must be a better way. Some ideas:

• Have separate CompressedPng and UncompressedPng objects, with only either raw or idat, and try to use them throughout the code accordingly.
• Have one body as enum Compressed/Uncompressed/Both
• Have getters/setters on PngData that clear idat_data when raw_data is set or borrowed mutably.

[andrews05]

In addition to the improvements made in #158 above, there have been further improvements in #482 as optimisation is now performed on the PngImage rather than the PngData. There's probably still some tidy ups that could be done but I think the main concerns here are now resolved. (Correct me if I'm wrong @kornelski)