gtk3: Fix Cairo backend

With GTK3, the Cairo surface we get is for the whole window, which means
the automatic size inference from matplotlib#22004 gets the wrong size. For the
GtkDrawingArea, the Cairo context is aligned and clipped to the widget,
so nothing goes out-of-bounds. However, since the Cairo renderer flips
the origin using the height in the calculation (which is, for the
window, bigger than the drawing widget), everything is drawn lower than
it should.

lib/matplotlib/backends/backend_gtk3cairo.py

@@ -13,15 +13,19 @@ def on_draw_event(self, widget, ctx):
 
         with (self.toolbar._wait_cursor_for_draw_cm() if self.toolbar
               else nullcontext()):
-            self._renderer.set_context(ctx)
-            scale = self.device_pixel_ratio
-            # Scale physical drawing to logical size.
-            ctx.scale(1 / scale, 1 / scale)
             allocation = self.get_allocation()
+            # Render the background before scaling, as the allocated size here is in
+            # logical pixels.
             Gtk.render_background(
                 self.get_style_context(), ctx,
-                allocation.x, allocation.y,
-                allocation.width, allocation.height)
+                0, 0, allocation.width, allocation.height)
+            scale = self.device_pixel_ratio
+            # Scale physical drawing to logical size.
+            ctx.scale(1 / scale, 1 / scale)
+            self._renderer.set_context(ctx)
+            # Set renderer to physical size so it renders in full resolution.
+            self._renderer.width = allocation.width * scale
+            self._renderer.height = allocation.height * scale
             self._renderer.dpi = self.figure.dpi
             self.figure.draw(self._renderer)