The following happens when processing an `@import()` rule:
1. `_st_theme_resolve_url()` to resolve file
2. `insert_stylesheet()` to track file/sheet
a. take ownership of file/sheet (ref)
b. use file as key in `stylesheets_by_file` hash table
c. use file as value in `files_by_stylesheet` hash table
3. release reference to file
This leads to a refcount error when importing a file that
was already parsed before:
1. file start with refcount 1
2. `insert_stylesheet()`
a. increases refcount to 2
b. inserting into `stylesheets_by_file` *decreases* the
passed-in key if the key already exists
c. `files_by_stylesheet` now tracks a file with recount 1
3. releases the last reference to file
The file object tracked in `files_by_stylesheet` is now invalid,
and accessing it results in a crash.
Avoid this issue by reusing existing stylesheets, so we don't insert
a stylesheet that's already tracked.
As a side-effect, this also saves us from re-parsing the same file
unnecessarily.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7306
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3619>
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