Add support for using BLF to draw into an ImBuf image buffer.
Once the imbuf context has been set, draw calls for that font_id will draw into the image.
This works by binding an imbuf to BLF which is then used as the target when drawing.
```
with blf.bind_imbuf(font_id, imbuf):
blf.draw_buffer(font_id, text)
```
See the example in the Python API documentation for reference.
The following BLF API's have been added to support a Python context manager.
- `BLF_buffer_state_push`.
- `BLF_buffer_state_pop`
- `BLF_buffer_state_free`
Ref !135772
