Changing console video mode results in loss of scrollback buffer contents.

[Darren Pilgrim]

I use the allscreens_flags to switch to higher-resolution, non-raster mode
at boot.  On a May 22 -current, the scrollback buffer was intact after
changing the resolution.  On -current as of yesterday, when I change console
resolution, the scrollback buffer is lost.  The new behaviour is equivalent
to issuing `vidcontrol -C`.

Was this change intentional?