Draw(2) and loading

Hello again,

I finally got it working!  My simple test draw(2) programs can draw on
the screen, but non-trivial clients still fail for various reasons, in
part because I'm having to heavy-handedly unmount all of /dev to wipe
out any traces of rio.

This week, I'm working on the yY requests (
/sys/src/libmemdraw/cload.c
/sys/src/libmemdraw/load.c
/sys/src/libmemlayer/load.c
) to load image(6) pixel data into a screen image.

So far I have a generic implementation that is dog slow and doesn't
actually work correctly, so I will have to fix its bugs and possibly
single out some special cases such as the one-bit images used for most
fonts.  It also only handles scaled values and has no idea what to do
with colour-mapped images.

I'm on vacation this week and my connection is spotty, but it can't
get any worse than the last two weeks of silence on my part!

-- David