Add doc for wxGraphicsContext.Create(), which is a lightweight context with no target, and which can be used for measuring text, etc.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74227 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Add some missing wxGraphicsContext methods

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74228 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775