Opening a JPEG now estimates the quality it was saved at from its quantisation
tables (the Annex K tables of ITU-T T.81 scaled by the IJG quality curve; exact
for such tables, the closest fit otherwise). Opening a WebP records whether it
is lossless (VP8L) or lossy (VP8), and a lossy one's quality from its base
quantiser index. The settings live on the document (`Document::file_encoding`,
saved in .pcraft) and a save that asks for no quality writes the file back the
same way, as Photoshop's Save does: File › Save, Save As and Save a Copy start
there instead of the fixed codec defaults (quality 90, WebP always lossless).
An explicit quality still wins; new documents and other formats keep the
defaults. Hostile or truncated DQT/VP8 headers give no estimate, never a panic.
Refs #2399
Co-authored-by: Claude Opus 5.5 <[email protected]>
