Implement bounding box computations for wxGDDC.

Update the bounding box in all the methods drawing something. This wasn't
done before in many of them, resulting in the bounding box remaining empty,
but it is updated now and a new test checking that it is was added.

Closes #12904.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76953 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775