allow RGB inputs for heatmap

[ffreyer]

Description

Fixes #2033

Adds support for heatmap(xs, ys, img) (back?)

Type of change

New feature / Bug fix

Checklist

• Added an entry in NEWS.md (for new features and breaking changes)
• Added reference image tests for new plotting functions, recipes, visual options, etc.

[MakieBot]

Compile Times benchmark

Note, that these numbers may fluctuate on the CI servers, so take them with a grain of salt.

using time

master:  8.76 < 8.81 > 8.86, 0.04+-
pr:      8.74 < 8.76 > 8.86, 0.04+-
speedup: 0.99 < 1.00 > 1.01, 0.01+-
median:  -0.56% => invariant

This PR does not change the using time.

ttfp time

master   24.69 < 24.87 > 25.11, 0.13+-
pr       24.77 < 24.84 > 25.01, 0.10+-
speedup: 0.99 < 1.00 > 1.01, 0.01+-
median:  -0.10% => invariant

This PR does not change the ttfp time.

[ffreyer]

All test failures are because I added a new refimg

[juliohm]

Thank you @ffreyer for fixing the issue, appreciate it.

[juliohm]

Can this PR be merged?

[ffreyer]

Yes, it's ready to be merged. I just need someone to review it

[juliohm]

@SimonDanisch can you please take a look at this PR?

[juliohm]

This PR contains a hot fix for an important issue. Appreciate if someone can review it.

Thank you again @ffreyer for the quick fix.