Remove more unnecessary headers from drawing test.

Don't include GUI headers in non-GUI code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76169 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Use "virtual" keyword with overridden methods.

No real changes, just make the code more clear by explicitly using "virtual".

Closes #16097.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76170 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775