st/texture-cache: Cancel pending requests on icon-theme changes

As outlined in commit 36b8dcbe, we can end up with wrong icons
if the icon theme changes right after a GTK theme change to/from
HighContrast triggered a theme reload.

That's because when we reload icons for the new icon theme, there
are already pending requests due to the icon-style change; those
requests are simply re-used for the new icons, with the existing
icon infos from the old theme.

The above commit applied a simple work-around by changing the
icon theme before the GTK theme, but that only works for the
HighContrast switch in our own UI.

It turns out that Settings also uses the "wrong" order, so the
issue still reproduces with the Universal Access panel.

So instead of relying on everything changing the settings in the
order we expect, cancel all ongoing requests on icon-theme changes.

GNOME/gnome-shell!1277
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