Blender Git Loki

Fix T46236: NLA transition strips do not get resized when neighbouring strips change

Transition strips in the NLA should always stick to whatever strips are beside it,
and are allowed to grow/shrink as needed to achieve this. Previously the code here
was only checking if the neighbouring strips started encroaching on the transition,
but not whether the transition needed to grow to fill a gap. It was also just
moving all strips when there was insufficient space, even though that would alter
timing down the track.

Now transition strip resizing works as follows:
* It will grow/shrink as necessary to absorb any changes in the length of its neighbours
instead of shunting everything around to maintain its length
* If the neighbour has been resized by an amount greater than the transition's length,
all the strips will need to be shunted away to make way for the neighbour. In this
case, the transition will shrink down to being 1 frame long to ensure that it is
still visible (so that it can be removed if necessary).