gibaseinfo: Fix use of stack-allocated GIBaseInfos

Most of the code for handling stack-allocated infos was correct, it was just missing code to initialise the GTypeInstance member.

Since GTypeInstance isn’t really designed for stack allocation, this is a little hacky — it requires setting up the member within GTypeInstance manually. It works, though.

The externally visible consequence of this, though, is that stack-allocated GIBaseInfos now need to be cleared when they’re finished being used. This allows the GTypeClass ref to be dropped.

All users of the stack-allocated APIs in libgirepository will need to adapt to this change.

Signed-off-by: Philip Withnall pwithnall@gnome.org

Fixes: #3218 (closed)