perf: calc theme color work is done during paintTile

[mmeeks]

I expect that these events are elided above us anyway, since paintPartTile should not be doing any event notifications but this:

Shows 5.5% of a spreadsheet editing being down to doing slow-looking things with theme color palettes at a poor time =)

The complete trace is here: https://github-production-user-asset-6210df.s3.amazonaws.com/833656/305063342-9140ce64-c2ff-46d0-80e1-8854b244859c.svg

@caolanm FYI :-)

Thanks! =)