Don't fail on equal-but-differently-named cmaps in qt figureoptions.

Currently, opening the Qt figureoptions UI for an image whose cmap is
not registered in the colormap registry, has a name not matching any
registry entry, but is actually equal (`==`, i.e. has the same LUT and
colorbar-extension attributes) to a registry entry, leads to an error.
A typical example would be
```
import cmap, pylab as p  # third-party
p.imshow([[0, 1]], cmap=cmap.Colormap("bids:magma").to_mpl())
```
and opening the qt figure options; this leads to the error "index
'bids:magma' is invalid ...".

Note that if the cmap is different from any registered
cmap then we already add it to the UI combobox (try e.g.
`cmap=cmap.Colormap("imagej:fire")`); the only problem was if it was
equal to a registered cmap (this arises because when the code was
originally written, cmap instance equality was by identity, not by
comparing LUTs, so the `cmap not in cm._colormaps.values()` check
behaved differently).

Fix that by checking whether the colormap *name* is registered.  The
behavior is still ill-defined in the opposite (theoretical) case of an
unregistered cmap different from any registered cmap but with a matching
name, but I'd argue that case is more pathological.

Test by running the above code and opening the qt figureoptions.

Author: anntzer

lib/matplotlib/backends/qt_editor/figureoptions.py

@@ -149,7 +149,7 @@ def prepare_data(d, init):
     cmaps = [(cmap, name) for name, cmap in sorted(cm._colormaps.items())]
     for label, mappable in labeled_mappables:
         cmap = mappable.get_cmap()
-        if cmap not in cm._colormaps.values():
+        if cmap.name not in cm._colormaps:
             cmaps = [(cmap, cmap.name), *cmaps]
         low, high = mappable.get_clim()
         mappabledata = [