vga: add elbow room for smooth scrolling.
vga: add elbow room for smooth scrolling.

Some high-performance smooth scrolling algorithms do best when allowed
to draw over the left and right edges of the raster area.  The library
now permits up to 16 pixels of overdraw in each direction.

cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib,cbiffle/m4vgalib