Editing has been simplified by breaking down each edit into a
combination of three basic single-element edits: MOVE, TRIM_START, and
TRIM_END.
Each edit follows these steps:
+ Determine which elements are to be edited and under which basic mode
+ Determine which track elements will move as a result
+ Snap the edit position to one of the edges of the main edited element,
(or the edge of one of its descendants, in the case of MOVE), avoiding
moving elements.
NOTE: in particular, we can *not* snap to the edge of a neighbouring
element in a roll edit. This was previously possible, even though the
neighbour was moving!
+ Determine the edit positions for clips (or track elements with no
parent) using the snapped value. In addition, we replace any edits of
a group with an edit of its descendant clips. If any value would be
out of bounds (e.g. negative start) we do not edit.
NOTE: this is now done *after* checking the snapping. This allows the
edit to succeed if snapping would cause it to go from being invalid to
valid!
+ Determine whether the collection of edits would result in a valid
timeline-configuration which does not break the rules for sources
overlapping.
+ If all this succeeds, we emit snapping-started on the timeline.
+ We then perform all the edits. At this point they should all succeed.
The simplification/unification should make it easier to make other
changes.
Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/issues/97
Fixes https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/issues/98
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/169>
This is implemented on top of a Tree that represents the whole timeline.
SourceClips can not fully overlap anymore and the tests have been
updated to take that into account. Some new tests were added to verify
that behaviour in greater details
