Use outlines for mesh selections instead of highlight colors

[Wumpf]

Uses outlines for selection/hovering of meshes.

mesh.outlines.sd.mov

Introduces a sibling data structure to SpaceViewEntityHighlight in order to quickly get the desired grouping-aware outline mask indices, called SpaceViewOutlineMasks. SpaceViewOutlineMasks will likely fully replace SpaceViewEntityHighlightin the near future; in any case a bit more work in this area is needed; consider this a first quick-and-dirty step!

Color choices etc. are not considered final, but a first step. We'll go over them again when everything is changed to outlines instead of highlights.

Copy pasting a few other notes I posted previously on the internal Slack:

*Previously, we distinguished “sibling selections”, i.e. you select something in one space view and there is a different (!) highlight in another space view where the same object is visible. That’s gone now because I can only support 2 channels at the moment

• (hardly visible in video) when an object outline is “see through” is not always intuitive in particular in regards to hover highlights. This happens because the the outline mask texture uses the same depth buffer for both selection & hover (but then computes different distance/voronoi values for each). In my humble defense, Blender has the exact same issues if one pays a lot of attention (but they don’t have hover, alas though a primary vs secondary selection which)
• our selection color is awfully dark. I had to brighten it up a bit but it still feels to dark. This means I need to make the lines thicker than I want to (the outline impl is fairly good with large outlines, but I didn’t actually want those)
• I’m again not happy with the anti-aliasing quality on non-4k screen. To be revisited. Might also just hint at me being obsessed at this point
• giving the hover outline some additive transparency feels really nice
• oh my god this so much better, the bar was low tho

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)

part of #889