VSE: Add channel headers

This patch adds channel region to VSE timeline area for drawing channel
headers. It is synchronizedwith timeline region. 3 basic features are
implemented - channel visibility, locking and name.

Channel data is stored in `SeqTimelineChannel` which can be top-level
owned by `Editing`, or it is owned by meta strip to support nesting.
Strip properties are completely independent and channel properties are
applied on top of particular strip property, thus overriding it.

Implementation is separate from channel regions in other editors. This
is mainly because style and topology is quite different in VSE. But
also code seems to be much more readable this way.

Currently channels use functions similar to VSE timeline to draw
background to provide illusion of transparency, but only for background
and sfra/efra regions.

Great portion of this patch is change from using strip visibility and
lock status to include channel state - this is facilitated by functions
`SEQ_transform_is_locked` and `SEQ_render_is_muted`

Originally this included changes in D14263, but patch was split for
easier review.

Reviewed By: fsiddi, Severin

Differential Revision: https://developer.blender.org/D13836

1 files changed, 5 insertions, 0 deletions

diff --git a/source/blender/sequencer/SEQ_render.h b/source/blender/sequencer/SEQ_render.h
index fcf9f2365e1..bfe10d3eae9 100644
--- a/source/blender/sequencer/SEQ_render.h
+++ b/source/blender/sequencer/SEQ_render.h
@@ -104,6 +104,11 @@ struct StripElem *SEQ_render_give_stripelem(struct Sequence *seq, int timeline_f
 
 void SEQ_render_imbuf_from_sequencer_space(struct Scene *scene, struct ImBuf *ibuf);
 void SEQ_render_pixel_from_sequencer_space_v4(struct Scene *scene, float pixel[4]);
+/**
+ * Check if `seq` is muted for rendering.
+ * This function also checks `SeqTimelineChannel` flag.
+ */
+bool SEQ_render_is_muted(const struct ListBase *channels, const struct Sequence *seq);
 
 #ifdef __cplusplus
 }