Fix deadlock in cairo-scaled-font.c

When cairo_scaled_glyph_page_cache needs to remove entries, cairo-cache calls _cairo_hash_table_random_entry() with the predicate _cairo_scaled_glyph_page_can_remove(). This function checks that the glyph_page scaled_font is not locked by testing scaled_font->cache_frozen. The scaled font is locked in the cache-cache destroy entry callback: _cairo_scaled_glyph_page_pluck().

There is a race condition here between testing scaled_font->cache_frozen and locking the font. Fix this by adding a new CAIRO_MUTEX_TRY_LOCK mutex operation, and using it to test and lock the scaled font in _cairo_scaled_glyph_page_can_remove().