Dropping a transition on the timeline failed with "the track has no
frame rate": drop_transition_at asked sequence_time_base for the TRACK
node, but the frame rate lives on the sequence (tracks carry no video
params, so the lookup always returned None).
With that fixed the drop deadlocked instead: the edge resolution ran
while holding the project lock, and the add_transition_at_seam/edge
builders lock the project internally. The function now plans under the
lock and executes after it is released.
Covered by engine_drops_a_transition_at_a_clip_edge: a head-edge drop
with no previous clip lands a single-sided transition wired into the
clip only.
Mike-Solar
5b0f5939f3