Introduce a dynamic coefficient for touchmove pan (yaw/pitch change) speed

With a fixed coefficient affecting touchmove pan speed, at large
values of config.hfov (zoomed out) panning will feel "sluggish"
due to panorama panning less than finger is moved on screen.
On the other hand, at small config.hfov values (zoomed in),
panning will be perceived to move faster.

To improve usability at both small and large config.hfov
(zoomed-out and zoomed-in) we introduce a dynamic coefficient
affected by the config.hfov value.

Currently this seems to roughly keep initial drag/pan start position
close to the user's finger while panning regardless of zoom level.

Enable additional URL configuration parameters.

Now the following can also be set from URL:
  - minHfov, maxHfov
  - minPitch, maxPitch
  - minYaw, maxYaw

The motivation for this was making it easier to create auto-generated
panoramas using Jekyll/Hugo and other static site generators without
needing to create JSON files for panoramas.

If a given panorama is not a full 360x180 panorama, then the above
parameters are needed in addition to haov and vaov parameters
in order for the user not to see black areas in the panorama.
Hence suggesting adding URL support.

Add support for URL specified firstScene.

This allows references to the standalone viewer to start in the
"middle" of a tour.