Make subsurfaces for X11 as follows:
- toplevel
- override-redirect
- moves with the parent
Attempt to use subsurfaces for popovers again
This does not quite work yet

Otherwise, overridden pick vfuncs don't work and we e.g. pick widgets
scrolled away in a viewport.

No need to keep a separate current_event variable around, we don't use
it anyway.

These signals are going away, don't mention them in
the docs.

We can use the existing ::event handler for this.

This fixes hidpi blurred outset shadows

We have a couple of Python 3.x scripts that parse C files, and since C
does not have any encoding, we need to force one ourselves, to avoid the
case when we're running the build in a non-UTF-8 locale.

https://bugzilla.gnome.org/show_bug.cgi?id=792497

GtkGesture is a GtkEventController. gtk_event_controller_dispose() calls
_gtk_widget_remove_controller(). That NULLs the pointer-to-Controller in
our EventControllerData but does not delete said ECData from our GList.

Subsequently, if that same Widget gets unparent()ed, that method calls
unset_state_flags(), which leads to doing reset_controllers() if we are
insensitive. Now, unlike most most other loops over the GList of ECData,
reset_controllers() does not skip nodes whose pointer-to-Controller is
NULL. So, we call gtk_event_controller_reset(NULL) and get a CRITICAL.

This surfaced in a gtkmm program. The Gesture is destroyed before the
Widget. The Widget then gets dispose()d, which calls unparent()… boom.
I didn’t find an MCVE yet but would hope this logic is correct anyway:

The simplest fix is to make the loop in gtk_widget_reset_controllers()
skip GList nodes with a NULL Controller pointer, like most other such
loops, so we avoid passing the NULL to gtk_event_controller_reset().

In other, live cases, _gtk_widget_run_controllers() loops over the GList
and removes/frees nodes having NULL Controllers, so that should suffice.
But this clearly was not getting a chance to happen in the failing case.

https://bugzilla.gnome.org/show_bug.cgi?id=792624