Currently, gtk3 widget backgrounds are not being applied (which only
matters if the figure patch is partially transparent). The lines added
by this PR are equivalent to the call to painter.eraseRect(self.rect())
in the qt5agg backend (compare http://doc.qt.io/qt-5/qpainter.html#eraseRect
and https://developer.gnome.org/gtk3/stable/GtkStyleContext.html#gtk-render-background).

As an example, run

import os; os.environ["MPLBACKEND"] = "gtk3cairo"
from pylab import *
from gi.repository import Gtk
rcParams["figure.facecolor"] = (0, 0, 0, 0)
css = Gtk.CssProvider()
css.load_from_data(b"* {background-color: red;}")
gcf().canvas.get_style_context().add_provider(
    css, Gtk.STYLE_PROVIDER_PRIORITY_USER)
gca()
show()

Without this patch, the red background is not drawn. (gtk3agg behaves in the same way after #10297 is applied).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way