Image allocation works

Hello again,

The b request to /dev/draw/n/data now allocates a new image, and I'm
working on the L request to draw a line.  I'm also going to start work
on implementing the r8g8b8a8 image format for the y request.

I should be able to draw a line on the screen fairly soon, and once
that works I'll start working on other simple operations like
polygons and ellipses and their associated test programs.

-- David