... one of my students is drawing lots of images with mirrored counterparts. it would be great to
be able to use a negative width or height in image() for that:

image( img, 0, 0 );
image( img, img.width, 0, -img.width, img.height );

won't be doing this because it would cause real havoc on the internals of
how images are handled (since it's not just image, it's get, set,
imageMode, etc).

scale(-1, 1) and imageMode(CENTER) should do the trick for now.