[Refactor] Break QGFImageFile's _save function into smaller pieces

[elpekenin]

Description

Makes it somewhat easier to read and should prevent errors like the one on CI for #20101

Tested converting a GIF with and without the changes, and diff didnt find any changes

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[elpekenin]

Could return a tuple here (return bbox, graphic_data, image_data, use_delta_this_frame, use_raw_this_frame) But i dont like it because (1) is a very long line (2) prone to errors if we re-order (read: add new) things

[github-actions]

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