fix wrong listing of fields in field table caused by last commit

[danieldreke]

My last commit caused a bug :(
The listed fields in field table are for certain cases wrong.
The changes in this commit seem to fix it for my scenario, but haven't checked other scenarios.
If you don't have to time to check, please undo my last pull request.
If you have time to check and if this commit fixes it, please pull.
Thank you.

[borysiasty]

Hi, I reverted that last commit right after merging it. Did you read my comment there?

Your approach works fine with rasters. But for vectors it reorders fields and mixes fields from different layers. So I'd order input layers rather than fields.

But in general I strongly recommend switching to the 'Sample Raster Values' processing algorithm. I only keep this plugin alive because of lack of such algorithm for sampling vectors.