Apply reversescale in contour and histogram2dcontour legends

[archmoj]

Expecting contour legend to flip fill direction when reversescale is set.
demo

[archmoj]

Similar bug in histogram2dcontour: demo
cc: @alexcjohnson

[etpinard]

Nice catch @archmoj !

We could release this in a patch, so no need to pin this to the v1.52.0 milestone.