Make GetClippingBox() work for wxPrinterDC in wxGTK.

Make GetClippingBox() work for wxPrinterDC in wxGTK.

GetClippingBox() implementation relies on wxDCImpl::m_clip[XY][12] being
updated in DoSetClippingRegion() but this wasn't done here. Fix this by adding
the code to do this to the base class version of this method and calling it
from wxGtkPrinterDCImpl.

Also, refactor wxGCDCImpl to reuse the same code instead of duplicating it.

Closes #14697.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72674 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775