Allow swapping the underlying rendering backend.

This patch implements backend_inline.set_rendering_backend, such that
`set_rendering_backend("foo")` behaves as if calling
`matplotlib.use("foo")`, which allows selecting an alternative rendering
backend.

In matplotlib itself, the only relevant alternative rendering backend is
cairo, which is generally less feature-ful than agg, but there are also
third-party backends, such as mplcairo
(`set_rendering_backend("module://mplcairo.base")`) which provide e.g.
improved text rendering for complex scripts (Arabic, Hebrew) and
different compositing options.

matplotlib_inline/backend_inline.py

@@ -3,10 +3,10 @@
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the BSD 3-Clause License.
 
+import importlib
+
 import matplotlib
 from matplotlib import colors
-from matplotlib.backends import backend_agg
-from matplotlib.backends.backend_agg import FigureCanvasAgg
 from matplotlib._pylab_helpers import Gcf
 from matplotlib.figure import Figure
 
@@ -18,6 +18,29 @@
 from .config import InlineBackend
 
 
+def set_rendering_backend(backend_name):
+    """
+    Set the rendering backend.
+
+    Parameters
+    ----------
+    backend_name : str
+        A backend name, as would be passed to ``matplotlib.use()``.  The
+        backend should be non-interactive.
+    """
+    global _backend_module, FigureCanvas
+    _backend_module = importlib.import_module(
+        backend_name[9:] if backend_name.startswith("module://")
+        else f"matplotlib.backends.backend_{backend_name.lower()}")
+    # Changes to matplotlib in version 1.2 requires a mpl backend to supply a
+    # FigureCanvas.  See https://github.com/matplotlib/matplotlib/pull/1125
+    FigureCanvas = _backend_module.FigureCanvas
+
+
+_backend_module = FigureCanvas = None  # Will be set by the call below.
+set_rendering_backend("agg")  # The default rendering backend.
+
+
 def new_figure_manager(num, *args, FigureClass=Figure, **kwargs):
     """
     Return a new figure manager for a new figure instance.
@@ -33,7 +56,7 @@ def new_figure_manager_given_figure(num, figure):
 
     This function is part of the API expected by Matplotlib backends.
     """
-    manager = backend_agg.new_figure_manager_given_figure(num, figure)
+    manager = _backend_module.new_figure_manager_given_figure(num, figure)
 
     # Hack: matplotlib FigureManager objects in interacive backends (at least
     # in some of them) monkeypatch the figure object and add a .show() method
@@ -152,12 +175,6 @@ def flush_figures():
         show._draw_called = False
 
 
-# Changes to matplotlib in version 1.2 requires a mpl backend to supply a default
-# figurecanvas. This is set here to a Agg canvas
-# See https://github.com/matplotlib/matplotlib/pull/1125
-FigureCanvas = FigureCanvasAgg
-
-
 def configure_inline_support(shell, backend):
     """Configure an IPython shell object for matplotlib use.