fix #1969, interpolate cmap correctly

[SimonDanisch]

This is really weird, I don't see any reason why texture(cmap, val01) should do incorrect interpolations when only having few colors in the colormap (e.g. like with the default one for image which is [:black, :white]).
Would be nice to figure out the exact bug here, but meanwhile this should fix it.
I also deleted the GLVisualize remnant of having a stroke_width for the heatmap.. This isn't used nor tested and only implemented in GLMakie. I feel like we should either support it fully, or just don't offer it.
If this makes it a breaking change, I may also just bring it back 🤷
Fixes #1969

[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:  12.16 < 12.64 > 12.84, 0.25+-
pr:      12.34 < 12.57 > 13.15, 0.31+-
speedup: 0.95 < 0.98 > 1.02, 0.03+-
median:  -0.49% => invariant

This PR does not change the using time.

ttfp time

master   31.67 < 31.80 > 32.94, 0.46+-
pr       31.07 < 31.90 > 33.03, 0.71+-
speedup: 0.97 < 1.01 > 1.03, 0.02+-
median:  +0.31% => invariant

This PR does not change the ttfp time.

[SimonDanisch]

Before:

This PR:

[IanButterworth]

Nice! Thanks 🙏