Apply the keyframe pivot to flipbooks too.
Apply the keyframe pivot to flipbooks too.

Fixes #36. @tconkling, could you please verify and do an exporter
release?

mathieuanthoine/flump,mathieuanthoine/flump,funkypandagame/flump,funkypandagame/flump,mathieuanthoine/flump,tconkling/flump,tconkling/flump