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.
rast/text_10x16: better smooth scrolling support.

In cases where the text output is narrower than the display (current
scrollers) the background now gets filled in, instead of left untouched.
This eliminates an artifact when mixing text and graphics modes.

Support for text output *wider* than the display (using overdraw) allows
for truly smooth scrolling now.
rast: pixel-doubling rasterizers.