Get rid of SCREEN_ADR(), use screen_adr() instead, which uses only BIOS
variables. Use vga_write*, vga_read*, etc. functions to access VGA memory,
and adjust those a bit. Cleaned up set_video_mode, but just slightly.


git-svn-id: https://dosemu.svn.sourceforge.net/svnroot/dosemu/trunk@1159 7758b857-130e-0410-ab9c-d8735a528d18