scroll-related fields into a struct so vertical and horizontal can be
handled by the same code; add to board_zoom out params indicating
whether further zooming is possible.
their games being drawn (no BoardActivity around) break the two
android-only callbacks out of UtilCtxt and into a new JNIUtils
interface that then requires new handing in C.
game_receiveMessage(), the latter combining
comms_checkIncomingStream() and server_receiveMessage() which are
always used together and require a stream be passed between which
currently can't happen across jni.