[Feature] Use colours provided by bg-atlasapi to render annotation labels

[alessandrofelder]

Is your feature request related to a problem? Please describe.
Some atlases, such as the Allen Mouse, provide colours for each of the regions of the annotations. We currently use these to colour the meshes, but not the annotation.

Describe the solution you'd like
We could use DIRECT colour mode for the napari labels layer to map the atlas colours to the annotation.

Describe alternatives you've considered
n/a

Additional context
Might be good to double-check rendering performance in napari if/when we implement this. See napari/napari#6397 - @Czaki kindly warned us that the fix for this may not help with DIRECT color mode.

[Czaki]

the napari/napari#6439 is fix for direct color mode

[alessandrofelder]

Ah, even better, then :) thank you @Czaki !