Initial implementation of f-curve drawing for the video sequencer.
The patch adds a flag in the VSE's View menu (active by default) for drawing opacity and volume f-curves as darker regions of the strips.
A similar solution could also be used for the NLA editor's influence curves. (currently drawn as gray lines).
To get better performance:
- Only the visible sections of the curve (inside the editor borders) are evaluated.
- The amount of frames for which the curve is evaluated depends on the zoom level.
- Avoid adding verts for points next to each others that have the same value
Further performance improvements could probably be achieved in curve lookups.