Skybox.backgroundTexture is ignored when blurBackground is true

[dmnsgn]

Issue: if backgroundBlur > 0, texture

https://github.com/pex-gl/pex-renderer/blob/5cced079d6b6330cf42656c57200d2c49e517806/skybox.js#L112-L128

TODO v4: add back backgroundTexture support (added here https://github.com/pex-gl/pex-renderer/commit/77cffc9bddaab73e2fc56014dccce5e146bdbbed) and handle above issue here:

https://github.com/pex-gl/pex-renderer/blob/90a39cb5b751e67eb8390aacff0ccb2bbcaa81c8/systems/renderer/skybox.js#L92-L100

[vorg]

I would consider it legacy feature as i haven't used it in a while. A better way to achieve it would be to have a way avoid rendering skybox which we currently hack manually in nodes.

[vorg]

Removed in favour of uBackgroundBlur