Support parallel requests for multiRes panorama tiles

* 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.

It never stops calling `animateMove`, if `pitch`, `yaw`, or `hfov` has not been changed.
Since we may have callback function, `animateMove` has to be called anyway, even if we don't actually need to animate movement. That's why equality of start and end positions is checked here, but not in `setPitch`, `setYaw`, and `setHfov`.

If `config.yaw` is close to -180 or 180, `yaw` might be out of bounds.

It's a common situation to remove an event listener inside its definition. Like this:
```javascript
viewer.on('load', function foo() {
	//code
	viewer.off('load', foo);
});
```

In this case `off` function removes the item from `externalEventListeners` array, while looping through this array may be not finished in `fireEvent` function. A reverse iteration fixes it.
Since we'd like to fire listeners in the order they were added, we need to use `externalEventListeners[type].length - i` index.

Fix removing event listeners / Fix 'setTimeout'