rw::Frame lacks getLTM()

[quiret]

In Criterion RW there is the RwFrameGetLTM function that returns the updated local transformation matrix from a Frame. This is very useful so that you dont have to care about updating it or hacking around non-existant getters or setters. There is no such method in librw, making access to LTM hacky.

Please add such a method.

https://github.com/aap/librw/blob/master/src/rwobjects.h#L115