FIX: deal with negative canvas dimension in Qt

In some cases Qt may report having a negative height (and presumably width) in the case of minimizing windows / widgets. This causes issues at the Agg layer when the re-size triggers a re-draw which tries to get a renderer with a negative dimension (segfaults or OutOfMemory errors).
matplotlib · jenshnielsen · Mar 14, 2016 · Mar 14, 2016 · Mar 14, 2016 · 1540c98eb91c685f0a15e4de4de3b85611f73194
commit 1540c98eb91c685f0a15e4de4de3b85611f73194
diff --git a/lib/matplotlib/backends/backend_qt5agg.py b/lib/matplotlib/backends/backend_qt5agg.py
@@ -172,6 +172,9 @@ def draw_idle(self):
            QtCore.QTimer.singleShot(0, self.__draw_idle_agg)

    def __draw_idle_agg(self, *args):
+        if self.height() < 0 or self.width() < 0:
+            self._agg_draw_pending = False
+            return
        try:
            FigureCanvasAgg.draw(self)
            self.update()