Introduce variables for the display and screen in the video mode functions.
This cleans up the code somewhat, and prepares for some upcoming changes.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75187 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775