relayd: clarify viewer_get_next_index no rotation condition

relayd: clarify viewer_get_next_index no rotation condition

The condition identifying cases where a viewer_get_next_index must not
perform a viewer stream rotation is relatively tricky to comprehend.

Split it in two intermediate variables, and use those variables to
print debugging statements when conditions for not performing a
rotation match.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ieca2fbf879c8ed45c0b3aa6035ac32679b12dcdc