The Viewport Render Keyframes operator didn't include keyframes from
grease pencil animation or from mesh shape key animation (see T72861).
To find all related datablocks that could be considered as part of the
selected object, `BKE_library_foreach_ID_link(..., IDWALK_RECURSE)` is
used. To prevent relations like constraint targets from being visited,
the recursion is stopped when it finds a different object.
Reviewed By: mont29
Differential Revision: https://developer.blender.org/D6558
d49fa504fd