openimageio: fix py-openimageio's version going backwards

When yuri@'s commit http://svnweb.freebsd.org/changeset/ports/555420
bumped openimageio's version, the Makefile was seeing PORTREVISION=1,
but there is a derived port, py-openimageio, which also .include-s
openimageio's Makefile.

Change this main port to set its PORTREVISION with ?= and add a
comment to remind editors.

Approved by: portmgr@ (blanket, trivial regression fix)