In the NLA, draw the track background based on the strip's extrapolation
setting. Previously, this had no visual indicator; "Hold", "Hold
Forward", and "Nothing" were visually indistinguishable. Now "Nothing"
actually shows nothing, "Hold Forward" shows a dimly colored background
from the strip to its right, and "Hold" shows that in both directions.
Reviewed By: RiggingDojo, sybren
Maniphest Tasks: T97572
Differential Revision: https://developer.blender.org/D14836
2310daed3a