Incorrect starting point for closed curves results in "twist" in interpolation.

[dalboris]

Reported in: http://www.vpaint.org/forum/viewtopic.php?p=397#p397

The issue is that the "starting point" of the closed edge is not correctly placed, resulting in shortening artefact during the linear interpolation.

Could be fixed by either let the user specify a starting point, or implement a smart auto-placement of this starting (or both: let the user decide, if he wants this to be automatic or manual. This could be an application-level user preference, that could be overriden for specific cells).

Related, but independent: implementing a better interpolation scheme than the current linear interpolation would lessen the issue (but not fix it -- better manual/automatic correspondence between closed edges is important nonetheless)

[dalboris]

(Note: in this example, there's also a caching issue explaining why the boundary is "outside" the shape)