heretic/hexen: Fix automap background scrolling.

There was a bug where it was possible to keep moving the background
when the boundaries of the map were reached. This is because the code
to move the background was done in calls to AM_Drawer(), which is
fundamentally a bad idea. Some old commented-out code shows that this
was previously done in AM_Drawer (same location that scrolls the map
itself), but it was moved. Move it back.

Thanks to Chris Fielder for the bug report; this fixes #321.