Starter images; 'New' now brings up a selection dialog showing 'Starter'
images and color choices.)
New images can be given solid background colors (which the 'Eraser' tool
erases to).
Improved --usage output.
Added support for "--papersize help" to list papersizes available via libpaper.
Some Magic tools that apply an affect once per click-and-drag no longer
recalculate the effect on pixels that have already been affected (until mouse
button is released and clicked again).
