DONE

v1.6.0 (WIP)

add new options (config-only):
- rotation_amortizaton (0.91)
- rotation_speed (20)
fix glcanvas background color
- fixed by dynamically updating glcanvas color based on the fact if we successfully loaded a bg image
- transparent
  - works with bg image and effects
  - doesn't work with static color bg
- black
  - works with static color bg (no pattern effect)
  - doesn't work with image
bg image slideshow -> investigate bg slideshow works with fade
- try loading -bg1.$bgext
- if successful
  - cache and show it
  - start a setInterval, which
    - tries loading -bg$next.$bgext
    - if successful
      - cache and show it
      - increase $bgnext
    - if unsuccessful
      - reset $bgnext and show from cache
video background (use bgext=mp4) and have gamename-bg.mp4 available
rename extlink to openexclusively
rename extlink2 to extlink (also: options, README.md, bigbox3d.config.json.example)
vertically center the loading div
updated gl-matrix 2.3.2 to 3.4.3

show loading counter, e.g. "loading... (3 / 6)"
background image (as alternative to background color)
- see: https://www.ragbag.info/3DBB/box.html?h=285&w=215&d=27&b=aaa&bgfx=https://www.ragbag.info/3DBB/boxen/Insta/Ports_of_Call/PoC.gif&s=100&z=1&f=0&t=boxen/Insta/Ports_of_Call/PoC_3D.jpg
- new option: bgext (different file extension of the background image, e.g. gif)
- stay compatible to colored background options
- loading text visibility
add external link to top-right, but only if embedded

extended functionality with bigbox3d.php
- provides specific meta tags for sharing links in Facebook, Twitter etc.
- utilized bigbox3d.conf.json so that you can omit URL params like path, ext and bg
some replacing in opts necessary because Facebook alters URL parameters