Rotation fixes

[Jordi Gutiérrez Hermoso]

Thanks to Shai's help, I've managed to get almost where I want to be.
I've implemented in C++ what looks like a good replacement of
__calc_dimensions__, and I also worked on a few fixes on how the
rotation interface was working.

Patch attached. Or if it's easier, pull from my clone:

     http://bitbucket.org/jordigh/gnu-octave

I still have to actually use __calc_dimensions__ to make sure that the
fltk backend can't rotate 2d images 3-dimensionally, but I'm hoping
this will be a faster fix.

HTH,
- Jordi G. H.

rotation.patch
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