Since we now remove all theme nodes on a stylesheet change (ie.
StTheme's "custom-stylesheets-changed" signal) instead of only
invalidating them, those nodes may not be accessed anymore as soon as
"custom-stylesheets-changed" is emitted.
It turned out though that when comparing them to the newly generated
nodes in `st_widget_recompute_style()` using the
`st_theme_node_paint/geometry_equal()` functions, the properties of the
old nodes will still be accessed, causing a crash since the
CRDeclarations are already freed.
To fix that, keep the reference to the CRStylesheet, which owns the
CRDeclarations used by the theme nodes, around a bit longer, so it's
still possible to access the CRDeclarations inside the
"custom-stylesheets-changed" signal handler. This allows us to compare
the old theme nodes to the new ones since the CSS properties of both are
still valid.
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2709
(cherry picked from commit e06109c23ce2a6dd53d32c26a89ab46a952d7373)
118676bffb