Do not modify parameters in place

Do not modify parameters in place

When the apply_transform_to_point() and its relative variant
landed in Clutter 0.3, the initial approach was to modify the
passed vertex as an in-out parameter. This was later dropped
in favour of a more consistent out parameter.

Unfortunately, the implementation never changed: both methods
where modifying the passed vertex with the partial results of
the computations.

This commit copies the contents of the "point" ClutterVertex
argument inside a stack variable and, for good measure, constifies
the argument.

Thanks to Thomas Steinacher for catching this in Python.