Brush preview is not accurate if finer magnifications don't exist or annotation is restricted

[daniel-wer]

Context

I have a magnification-restricted annotation where mags 1 to 8 were removed from the annotation layer. Still, annotation is allowed while being zoomed in (due to the fallback rendering mechanism). Then I am using the brush tool while mag 4 is active and want to use the brush preview to show me which voxels I'm about to annotate.

Expected Behavior

The brush preview should be accurate and fully highlight all voxels that will be annotated in mag 16 once I use the brush at the current position.

Current Behavior

The brush preview is very detailed and has mag-4-resolution, although the finest mag allowed to be annotated is mag 16.

Steps to Reproduce the bug

• Cannot reproduce the bug anymore / needs deeper investigation.

1. Create an annotation restricted to mag 16+ for a dataset with a color and a segmentation layer
2. Activate the brush tool and zoom in to mag 4
3. The brush preview looks as if mag 4 voxels would be annotated whereas only mag 16 voxels will actually be annotated

The mag that is allowed to be annotated in should be supplied to the shader code, so that the brush preview can be rendered as if annotating in that mag instead of the current one. Will become more relevant with #8566