* Add scale option to hot spot

When `true`, the hot spot is scaled to match the fish eye effect of the image.

Hot spots then appear to be embedded in the panorama instead of floating on top.

Normally, the value of `minHfov` is ignored for said panoramas, and an
automatically calculated value is used instead.

This reverts commit 7e7f19e7e0.

This reverts commit 08ef73866a28d3e9334cd2852a40c8905b5d6b9e, reversing
changes made to 24e55d8f0b.

* support also partial cubemap/fallback images: backgroundColor for missing face(s)

* extend multires/generate.py to cylindrical input and partial panoramas

* fixed a few typos in new help strings of generate.py

* Missing cubemap faces now need to be defined as null in the configuration

* made generate.py strict again on existing output directory, printing descriptive error message

* replaced unsafe eval by ast.literal_eval in generate.py

* a couple of minor improvements regarding 'undefined' in libpannellum.js

* made generate.py allow existing output directory in debug mode

for convenience, no need to delete output directory every time while testing

* prevent display of out-of-range areas of partial (cylindrical) panoramas

by adaping min/may yaw and max horizontal fov, this no background (empty space) shown

* improvements as requested: made view restrictions optional, etc.

* extend documentation to describe new avoidShowingBackground option

Implementation of https://github.com/mpetroff/pannellum/issues/151.