gtk+ 3.22.6 release.

Bugs fixed:

• 774534 776132 776187 776012 774784 776187 776306 775808 776524 776560
• 774534 input shape and opaque region not applied without begin_paint()/end_paint()
• 774784 Failed to get desktop session proxy is not an error!
• 775808 win32 maximized window is larger than the extended screen in dual monitor
• 776012 GtkFlowBox, GtkListBox: Don't emit signals etc. during destruction
• 776132 Mention the difference between gdk_window_create_similar_image_surface and...
• 776187 flowbox: Add gtk_flow_box_get_child_at_pos to gtk3
• 776306 flowbox: Sometimes emits child-activated during rubberband selection
• 776524 GtkAboutDialog: Fix formatting of example email address in html documentation
• 776560 icon-browser: window opens at very narrow size, only showing 1 column of icons
• Fix GL checks to work better on OpenGL ES 2.0
• Avoid a possible crash in ::activate-url handlers
• scrolledwindow: Fix func summary being cut off in bindings using doxygen