libdraw determines the iounit in initdraw() and sizes
its write buffer accordingly. uploading pixels into a image
will split the y/Y operation in multiple ones so they fit
into the buffer.
Yes, and the data doesn't go through rio (unlike an older system), but to /dev/draw, so in the local graphics
case the kernel is pulling the draw data directly from the client memory: it doesn't go through 9p.
If /dev/draw is imported from a remote system, the local system will generate the 9p, and the remote
will receive that, turn round and access its /dev/draw.