Implement ROI_WPNEXT_OFFSET

[LorenzMeier]

@bkueng We've discussed this earlier but I dropped the ball - it's getting critical now. What we need is to implement the ROI_WPNEXT_OFFSET command which sets the gimbal angle in VMOUNT to a fixed offset in body frame (as provided by the command). So it means copying the angle value from ROI_WPNEXT_OFFSET in VMOUNT.

[LorenzMeier]

@thomasgubler FYI - this should be a quick win and @DonLakeFlyer needs this to get unblocked.