taowm: move offscreenXY away from the int16 underflow discontinuity.
taowm: move offscreenXY away from the int16 underflow discontinuity.

This works around a session-crasher occasionally seen when using the GIMP's
"Perspective" tool.

TBR=nigeltao
R=nigeltao
CC=taowm
https://codereview.appspot.com/219560044

nigeltao/taowm