Blender Git Loki

Implement asymmetric and free handles type for masks

Summary:
The title actually says it all, it's just possible to
have independent free handles for mask splines. Also
it's now possible to have aligned handles displayed
as independent handles.

Required changes in quite a few places, but they're
rather straightforward.

From user perspective there's one really visible change
which is removed Handle Type menu from the panel. With
asymmetric handles it's not clear which handle type to
display there. So now the only way to change handle type
is via V-key menu.

Rewrote normal evaluation function to make it deal
with new type of handles we support. Now it works in
the following way:

- Offset the original spline by maximal weight
- Calculate vector between corresponding U positions
on offset and original spline
- Normalize this vector.

Seems to be giving more adequate results and doesn't
tend to self-intersect as much as old behavior used to,

There're still some changes which needed to be done, but
which are planned for further patch:

- Support colors and handle size via themes.
- Make handles color-coded, just the same as done for
regular bezier splines in 3D viewport.

Additional changes to make roto workflow even better:
- Use circles to draw handles
- Support AA for handles
- Change click-create-drag to change curvature of the
spline instead of adjusting point position.

Reviewers: campbellbarton

CC: sebastian_k, hype, cronk

Differential Revision: https://developer.blender.org/D121