- Clear the mesh before setting it up again to prevent an assert when showing/hiding a slot image.
- Recalculate normals only if required by the SpriteCollection.
- Added Tangents generation if needed.
Merge branch 'master' of https://github.com/EsotericSoftware/spine-runtimes.git
Merge pull request #46 from Dgizusse/master

Small fix and optimization when updating the mesh.