Also add ability to provide a hook to execute just prior to rendering.

It doesn't make much sense to provide a URL to a non-WebGL viewer anymore.

Spherical harmonic transform (SHT) previews are display instead of a solid
background color before the base multires tiles are loaded. If a thumbnail is
provided, it will be displayed instead of the SHT preview. If the thumbnail is
provided as a Base64-encoded string, the SHT preview may be briefly displayed,
since image loading is asynchronous, while the SHT preview is loaded
synchronously.

* 64c337b

* Update build.

* Build 05837fd.

* Build d1555b3,

* Build eb41687.

* Build b232a6b.

* Build 233b100.

* Build ab7b859.

* Build 805f6da.

* Build 25e3d6d.

* Build 54a8dc6.

* Build 0d32eea.

* Build eeb9383.

* Add code.

* Improve/fix interaction message fade.
Fix indentations.

* Build for UI confirmation (40ffc37).

* Ignore build/ again.

* Change interation message text and style.

* Don't display interaction messages while in fullscreen.

* Don't change defaults and add documentation for interation messages.

* Disable `ctrl` / `shift` zoom when `ctrl` is required for mouse wheel zoom.

* Show interaction key based on platform.

* Cleanup.

* Remove pre-built files.

* Use `<div>` for interaction message.

* Fix case without drag confirm and show proper message for `both`.

Also switch back to using separate `dragConfirm` parameter.

* Document interaction message events.

Co-authored-by: Matthew Petroff <matthew@mpetroff.net>

Default to not doing so except for in standalone viewer.

* Hotspot dragging (hot spot attribute draggable + dragHandler function)
* Allow Image and ImageData and ImageBitmap as parameter panorama for equirectangular scenes

* 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.

* 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.