Instead query g_VideoMode where needed. As both g_xres and g_yres aren't
global anymore remove static from UpdateRenderer. While at it use more
desicriptive names: GetXRes -> GetWindowWidth, GetYRes ->
GetWindowHeight.
This reduces the amount of variables tracking width and height by one
each.
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>