Fix misaligned colors in colorbar?

[briochemc]

Description

Fixes #4121?

Currently the Colorbar built from a contourf may have misaligned colors. For a MWE see #4121 or

l = [1, 2, 5, 10, 20, 50]
x = 0:0.1:51
y = 0:0.1:51
z = [y for x in x, y in y]
fig, ax, plt = contourf(x, y, z; levels = l, extendhigh=:auto, extendlow=:auto)
cb = Colorbar(fig[1, 2], plt; tellheight = false)

which produces this misalignment (I edited the image in Preview to show the differences in colors):

The issue seems to be that the continuous_pixels (used to draw the heatmap for the colorbar) are not scaled properly. Simply adding the colorrange kwarg to that call fixes this issue for me, with properly matching colors:

However, I'm not if this breaks other things elsewhere, so hopefully someone can hold my hand to finish this PR?

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Added an entry in CHANGELOG.md (for new features and breaking changes)
• Added or changed relevant sections in the documentation
• Added unit tests for new algorithms, conversion methods, etc.
• Added reference image tests for new plotting functions, recipes, visual options, etc.