This is my attempt of adding defaults for the space clip editor struct
(in line with https://developer.blender.org/T80164).
It adds the default allocation for `SpaceClip` and
`node_composite_movieclip.cc`. This also solves the error below (for
C++ files using the DNA_default_alloc), which was put forward by
Sergey Sharybin.
Differential Revision: https://developer.blender.org/D13367
Reviewed by: Julian Eisel
bab47b60cb