Previously, the depsgraph assumed that every node tree might contain
a reference to a video. This resulted noticeable overhead when there
was no video.
Checking whether a node tree contained a video was relatively expensive
to do in the depsgraph. It is cheaper now due to the structure of the new
node tree updater.
This also adds an additional run-time field to `bNodeTree` (there are
quite a few already). We should move those to a separate run-time
struct, but not as part of a bug fix.
Differential Revision: https://developer.blender.org/D14957
f517b3a295