graphics/processing: pin to openjdk8

Dependency comms/rxtx is tied to openjdk8 and installs jars in
PREFIX/openjdk8/jre/lib/ext. This directory is gone in jdk9+, but
processing expects it.
So pin processing to openjdk8 also.
If anybody wants this to be supported by jdk9+, patches are welcome.

PR: 292652
Approve-by: maintainer timeout