Support matplotlib backend "agg" for CueFigure

"agg" is the terminal Linux backend, since the user should be aware that
nothing can be shown in this case and raising an error prevents testing
certain functionalities, we support it and do the same commands as for
the "inline" backend.

src/scwidgets/cue/_widget_cue_figure.py

@@ -67,6 +67,7 @@ def __init__(
         if matplotlib.backends.backend in [
             "module://matplotlib_inline.backend_inline",
             "macosx",
+            "agg",
         ]:
             # we close the figure so the figure is only contained in this widget
             # and not shown using plt.show()
@@ -104,6 +105,7 @@ def clear_display(self, wait=False):
         if matplotlib.backends.backend in [
             "module://matplotlib_inline.backend_inline",
             "macosx",
+            "agg",
         ]:
             self.clear_figure()
             self.clear_output(wait=wait)
@@ -131,6 +133,7 @@ def draw_display(self):
         if matplotlib.backends.backend in [
             "module://matplotlib_inline.backend_inline",
             "macosx",
+            "agg",
         ]:
             with self:
                 display(self.figure)