Crash on null dereference in `SEQ_timeline_boundbox()`. This function was
generalized in rB9e4a4c2e996c to work on arbitrary `seqbase`.
Fixed by refactoring `SEQ_timeline_boundbox()` functions to return default
sane values if `seqbase` is `NULL`
Reviewed By: HooglyBoogly
Differential Revision: https://developer.blender.org/D9878
Logic was broken into finding gaps and ofsetting strips.
Functions were modified so they work on explicitly defined seqbase,
so they can be used as python API functions.
Functional changes:
- Improve performance by calculating gap length and offseting strips
at once. Previously strips were offset by one frame.
- Calculate gap from start frame. Previously gap was considered only
inbetween strips.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D9730
Move most functions into separate files.
File sequencer.c is reserved for alloc, free and duplicating functions.
There should be no functional changes.
