Minor patch from David Megginson:

Minor patch from David Megginson:

Fix FGViewer::update so that pitch offset and goal pitch offset work
together nicely (the offset was snapping to 90/-90 when only one of
the two was changed).

Viewer improvements from Jim Wilson:

These files get the 3d cockpit working and fix a few issues in the viewer
code.
XYZ offsets are now defined as follows: X -left/right+ (along wing axis), Y
-up/down+ perpendicular to the aircraft, Z is -in/out+ the aircraft's body axis.
I've also done some cleaning up of unused and mostly unusable interfaces,
added commentary to the *.hxx, combined together some duplicate code and
eliminated a couple unecessary operations.  I also moved what was left of the
"protected" zone to "private" since we aren't subclassing this anymore.