Calculating purge volume doesn't properly account for both filaments in the transition

[cinnamennen]

Description of the bug

I am working on printing PETG w/ PLA supports. A key aspect of this is purging more when you switch between materials. While I can set the filament_purge_multiplier for my PLA support material, this only is accounted for when switching to the material, not from the material.

Project file & How to reproduce

1. Have a MMU project.
2. Set one of the filaments to a very high filament_purge_multiplier
3. Note that only transitions to that filament are increased

Checklist of files included above

• [X] Project file
• [X] Screenshot

Version of PrusaSlicer

2.8.0

Operating system

Windows 10

Printer model

Mk4 w/ MMU3