`blf_dir_search` BLF util would not properly handle relative fonts not
found in pre-defined 'system fonts' directoriesi stored in
`global_font_dir` global variable.
Now it rebases relative paths to current .blend file location as
expected.
Note: the fact that VSE is setting font ptaths relative by default is
probably not actually desired, but this is another issue really. See
`BKE_sequencer_text_font_load` code.
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