This resolves pixelation issues on some mobile devices (fixes #725).
It also may be related to issues #301, #496, #683, and #708.

For three event listeners added to the container elements weren't being
removed properly, since `removeEventListener` was mistakenly being called on
`uiContainer` instead. This was preventing garbage collection.

Add `stopMovement` to stop all movement

This allows for equirectangular images on a given device to be twice as big as
they were previously, so (almost) all devices now support equirectangular
images up to 8192px across.

This technique could be extended to double the maximum size again (using eight
textures), but images bigger than 8192px should be converted to the
multi-resolution format anyway, so there's little point in implementing this
extension.

Sometimes more than just the target attribute needs to be customized on the hotspot. Expansion of ba35ea8

Hotspots now accept an object with a list of attributes to append to the anchor tag.

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

Sometimes it is needed to open HotSpot link right in the same frame ("_self") or in a named one.

* 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

This reverts commit 2491c2b4dc.

Avoid loading invisible files for partial panos of any image type.