Move wxTransform2D methods' bodies to geometry.cpp.

They are all virtual and so cannot be inlined anyway and having them in
the header like this confuses the g++ linker into always pulling some of
the methods in merely because geometry.h was included.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73984 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775