A small patch to fixup some things and add a new </closeall> tag

New features:

- Added </closeall> tag which closes all open tags (suggested by @Alex from Alaska)

Bugfixes:

- Fixed default avatars not spawning properly (reported by @Zane)
- Fixed race condition breaking the font glyph generation, when the old atlas is unloaded while another thread is trying to pin glyphs in it
- Fixed tags without parameters being accepted when specifying arbitrary parameter
- Fixed Vive Pro Eye being repeatedly initialized when the SR runtime is not intsalled on the system