Prefetching happens on a background thread by design, and so it was not loading
any fonts and using the default monospace font.
Address this by making all font usage within VSE use "unique" BLF font objects,
and protecting concurrent access to them or their state within VSE itself:
- SeqFontMap structure to hold a mutex, file path -> font ID map (for file
based fonts), name -> font ID map (for datablock fonts).
- seq_load_font_file, seq_load_font_mem, seq_unload_font that use the above
instead of calling into BLF directly.
- Text effect rendering part guards with a mutex, so that font render state and
glyph cache does not get trashed.
- SeqFontMap is global instead of some Scene-specific runtime data (e.g.
scene->ed), because right now there's a hard max limit of total number of
fonts (64), and if each copy of the VSE data would start to load their own
unique fonts, that limit could get easily reached.
- BLF font loading/unloading code is now thread safe. The rest of BLF drawing
is still not!
If at some point in the future BLF font usage becomes thread safe, and font
drawing becomes "stateless", this whole machinery can be removed.
Pull Request: https://projects.blender.org/blender/blender/pulls/130542
11a99fb2b1