Move position of vertices along plane of sprite, and UV coordinates

[rantonels]

Currently single vertices can be moved orthogonally to their sprite but not in the other two directions. This would be a tremendously useful feature. My particular interest is in morphing tile-based meshes to make them slightly more organic.

In addition, the possibility to move the UV coords of single vertices with a similar interface would also be great. Could be used for avoiding texture stretching on slanted tiles for example, or for special effects (i.e. "truncating" a sprite).

It seems natural also that if these features are implemented they should really be paired with the possibility to flip the triangulation on a sprite.