Advanced save flow

[Tyriar]

With the new png and tga encoders, Luna Paint has the ability to fully control how images will be encoded. I imaginee there will be two types of saves:

• Simple (eg. save): Original chunks will be carried over as per the spec
• Advanced (eg. save as or separate command?): Full control over what gets saved in the file, allow specifying text chunks, compression, etc.

To compliment this there should maybe be a setting to strip out unneeded chunks such as Software keywords? For color modes/bit depth that will probably come later with indexed support.