Fix loading of reference image settings

On my last fix I stumpled upon this bug. The saturation for reference images gets saved correctly but on loading it was restored from the opacity value.

There doesn't seem to be a bug report for this.

Test Plan

Just load a document with a reference image that was saved with different values for opacity and saturation and verify they get restored correctly.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
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