Respect 'regions' kwarg for 2D Makie scalarplots

[pjaap]

This is WIP.

I want to pass the regions kwarg to the Makie backend.

To do this, I simply push only triangles into the list, that correspond to the region numbers.

Additionally, I want the colorbar to be fitted to the values on the desired regions. Therefore, I introduced :fit for limits to compute the extrema on that regions.

@j-fu

• Is this approach going in the right direction?
• how to compute the limits more efficiently?

TODOS:

• also for 1D/3D scalarplots
• CHANGELOG
• DOC (for :fit)
• ...