stitcher tab FoV/Size relation

HFOV, VFOV, width and height are related.

currently (SVN 2841):
- when I change VFOV, height changes
- when I change HFOV, height changes
- when I change width, the Crop changes
- when I change height, VFOV changes

I find this confusing.

I would like clearer, advanced notice over how these parameters interact. I am not sure I have thought everything through, but it seems to me that there are at least three potential scenarios:

[ -> = changes ]

* keep aspect ratio constant
-- VFOV <-> HFOV.
-- width <-> height.

* keep view constant (=> change resolution)
-- VFOV -> height.
-- HFOV -> width.
-- height <-> width.

* keep resolution constant (=> change view)
-- height -> VFOV
-- width -> HFOV
-- VFOV <-> HFOV

I am not sure if there are other scenarios. predictability is the key. sure, I could play around for half hour and learn the tricks, but it is more important imo for the interface to be intuitive right away.