gl: Avoid a use-after-free
This only happens with big, non-atlased glyphs, so it is rare, but it gets triggered by the Masking demo in gtk4-demo.
Fixes: #6347 (closed)
This only happens with big, non-atlased glyphs, so it is rare, but it gets triggered by the Masking demo in gtk4-demo.
Fixes: #6347 (closed)