The next commit is going to parallelize gimpbrush-transform using
the gimp_parallel_distribute_foo() functions.  To spare us a lot of
boilerplate code, we're going to use C++ lambdas as callback
arguments to these functions.

This commit does the initial conversion of gimpbrush-transform.c to
C++, renaming it to gimpbrush-transform.cc.  We do this in a
separate commit, so that the changes are small enough for git to
register this as a renamed file, rather than a new file, so that we
don't lose the file's history.