Fix handles disappearing with invertedAxis

[Hypnosphi]

This fixes #1432 by reusing BrushHelpers.getHandles

This also moves brushDimension checks to BrushHelpers.getHandles, to ensure that behaviour is consistent with cursors

[boygirl]

@Hypnosphi Thanks for this fix! I just took a look locally, though, and noticed a small regression. The top and bottom handles seem to be slightly offset from the edge of the brush area. I'm guessing there's a missing negative sign when accounting for the handle width.

This should illustrate what I mean:

Once that's fixed, this looks good to merge.

[boygirl]

Thanks!