NLA SoC: Operators for 'tweaking' strip actions (TAB-Key for both)

In this commit, I've introduced the mechanism by which actions already referenced by strips used in the NLA can be edited (or 'tweaked'). To use, simply select a strip you wish to edit, and hit that TAB key to start tweaking that strip's action, and hit TAB again once you're done.


What happens when you enter 'tweak mode':
1) The action of the active strip temporarily becomes the 'active action' of the AnimData block. You are now able to edit this in one of the Animation Editors (DopeSheet/Action, Graph Editors) as per normal (i.e. sliding keyframes around, inserting keyframes, etc.). The 'action-line' will therefore get drawn immediately above the active track containing the active strip, so that it's clear that that's what we're editing.

2) All the NLA-tracks (and all the strips within them) that occur after the track that the active strip lived in get disabled while you're in tweakmode. This is equivalent to travelling back to an earlier state in a construction history stack.

3) The active NLA track also gets disabled while in tweakmode, since it would otherwise interfere with the correct functioning of the tweaking for the action of interest.

4) The 'real' active action (i.e. the one displaced by the active strip's action) gets put into temp storage, and will be restored after you exit tweakmode.

5) Any strips which also reference the action being tweaked will get highlighted in red shading to indicate that you may be making some changes to the action which you don't really want to make for the other users too.


Please note though, that this is only a rough prototype of this functionality, with some niceties still to come. i.e.:
* NLA-tracks after the active track should still get drawn above the 'tweaking action line', but perhaps with different appearance?
* Various tools will still need awareness of this to prevent corrupting operations from taking place. How to proceed is still undecided...
* When exiting tweak-mode, the strip the action came from still needs some form of syncing with the modified action... there are a few tricky issues here that will need to be solved
* Evaluation code doesn't totally take this into account yet...

---
Also, fixed a number of bugs with various code (notably selection, and also a few drawing bugs)

1 files changed, 4 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_nla.h b/source/blender/blenkernel/BKE_nla.h
index 3c3abe11da3..89e5f1af7e1 100644
--- a/source/blender/blenkernel/BKE_nla.h
+++ b/source/blender/blenkernel/BKE_nla.h
@@ -59,9 +59,13 @@ void BKE_nlatrack_set_active(ListBase *tracks, struct NlaTrack *nlt);
 short BKE_nlatrack_has_space(struct NlaTrack *nlt, float start, float end);
 void BKE_nlatrack_sort_strips(struct NlaTrack *nlt);
 
+struct NlaStrip *BKE_nlastrip_find_active(struct NlaTrack *nlt);
 short BKE_nlastrip_within_bounds(struct NlaStrip *strip, float min, float max);
 
 void BKE_nla_action_pushdown(struct AnimData *adt);
 
+short BKE_nla_tweakmode_enter(struct AnimData *adt);
+void BKE_nla_tweakmode_exit(struct AnimData *adt);
+
 #endif