AP_Camera: FOV quaternion should be Earth-Frame

[robertlong13]

Gimbal attitude is reported in a weird "hypothetical earth-frame quaternion if you pointed the aircraft north", which I'm guessing is important for some historical compatibility reason. The gimbal attitude status message at least has a flag indicating if it's using this. The camera FOV status message does not have such a flag, as such I believe the quaternion should be the proper Earth-frame quaternion. You could check the gimbal attitude message, to see if the gimbal is using the "yaw is body-frame", but I think the FOV message should contain all the information needed by a GCS to turn pixel coordinates into a 3D ray vector.

In my testing during Mission Planner development, I'm using this dirty hack as a workaround:

    // getting corresponding mount instance for camera
    AP_Mount* mount = AP::mount();
    if (mount == nullptr) {
        return;
    }
    Quaternion quat;
    Location loc;
    Location poi_loc;
    if (!mount->get_poi(get_mount_instance(), quat, loc, poi_loc)) {
        return;
    }

    float roll, pitch, yaw;
    mount->get_attitude_euler(get_mount_instance(), roll, pitch, yaw);
    quat.from_euler(radians(roll), radians(pitch), radians(yaw) + AP::ahrs().get_yaw());

Platform
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