Avoid triggering full dpi change if it's unchanged.

This is essentially a revert of the `Text.get_window_extent` change to
its dpi handling, but applies in _all_ cases.

Depending on backend, we want to avoid this extraneous dpi change,
because it can cause recursive event signals. For example, with GTK3, a
resize will trigger its event loop, which eventually causes a re-draw.
The re-draw would get to `Text.get_window_extent`, change the dpi (to
the same value), and call `Figure.set_size_inches`, which would resize
the window again. Depending on how much of the GTK event loop was
processed, this may or may not trigger another draw, recursing again and
again.

Fixes matplotlib#16934.

Author: QuLogic

lib/matplotlib/figure.py

@@ -442,6 +442,9 @@ def _set_dpi(self, dpi, forward=True):
         forward : bool
             Passed on to `~.Figure.set_size_inches`
         """
+        if dpi == self._dpi:
+            # We don't want to cause undue events in backends.
+            return
         self._dpi = dpi
         self.dpi_scale_trans.clear().scale(dpi)
         w, h = self.get_size_inches()