Add Draw:image-size and don't upscale images in emails.

Add Draw:image-size and don't upscale images in emails.

New display function Draw:image-size reports the size of an image in
(x,y).
This is used when drawing an image with new attr noupscale to
ensure the size of the image is never scaled up, only down.

email now requests noupscale

Signed-off-by: NeilBrown <neil@brown.name>