Add incremental BK loading

[Vagabond]

On slow hardware, the blocking bk loading can cause the audio to stutter because it blocks the engine loop. This PR makes loading BK files incremental, so the game loop can keep ticking and loading the BK file incrementally during scene transitions.

[Vagabond]

One issue is, on scene transition without a "crossfade" it flickers a bit

[Nopey]

OMF logo to main menu crossfade repeats four times
Other cross fades seem unaffected

EDIT: OpenOMF.bk to intro.bk transition can also repeat four times

2024-11-07.13-07-03.mp4

[Vagabond]

OMF logo to main menu crossfade repeats four times Other cross fades seem unaffected

EDIT: OpenOMF.bk to intro.bk transition can also repeat four times
2024-11-07.13-07-03.mp4

This should be resolved now.