An installed family was added to the font database one file at a time, so a
lookup on another thread could find it with only some of its styles: text
falling back to Arial took Arial Bold Italic or Bold (its files sort first)
for upright Regular text, and the same text laid out a moment later took
Arial Regular. One Arabic word could even mix the two. Two threads loading
the same family could also add a face twice.
A family's files are now read and parsed first and their faces added under
one lock, skipping faces another thread added meanwhile. Loading a family
reports it loaded when another thread loaded it meanwhile, so the fallback
search doesn't go on to other families.
This made tests_bidi::a_change_of_paint_in_a_mixed_direction_line_moves_no_glyph
fail intermittently when another test loaded Arial at the same time.
Taha-Mahmoodi
6b52e54a1b