Fix prime tower when only raft has multi-material

[casperlamboo]

Description

We didn't take into account that raft layers are negative

CURA-11291

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)

How Has This Been Tested?

🏃🏼‍♀️ running from sause

Checklist:

• [x] My code follows the style guidelines of this project as described in UltiMaker Meta
• [x] I have read the Contribution guide
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have uploaded any files required to test this change

[github-actions[bot]]

Unit Test Results

26 tests  ±0   26 :heavy_check_mark: ±0   15s :stopwatch: -1s   1 suites ±0     0 :zzz: ±0    1 files   ±0     0 :x: ±0 

Results for commit 723f66e0. ± Comparison against base commit eeb93cfa.

:recycle: This comment has been updated with latest results.