APIs missing: flatten

[matteogratton]

An API command to flatten a Shape/ShapePath is missing.

Currently we should work with this suggestion:
#139

Flatten is not only a transform to a bitmap, but also a way to apply the changes to a layer and reset its bounding box. For instance, a rotated layer by 45degrees, once flatten lost the rotation value and it's frame takes the current absolute position and size.