In cbook.safe_mask_invalid also ensure that the data is in
native byte order.

close matplotlib#6671 closes matplotlib#6394

This maybe the source of flaky tests.

…cal-none-linestyle

Fix canonical name for "None" linestyle in qtconfig gui

TST: add missing decorator

FIX: handle non-native endian images

Fixes the first two points of matplotlib#5341.

This patch allows the user to interactively set an image's colormap from the
axes and lines (and now images) parameters editor (in the Qt backend).

Colormaps are listed from those registered with matplotlib.cm.  Colorbars seem
to be handled properly.

The previous implementation was incorrect and would yield a double entry
in the cmap combobox.

Currently, `(cmin, cmax)` is swapped if `cmin > cmax` because it's not
easy to impose the constraint using formlayout.