OpenXR 1.0.6

This is a patch release.

Note: This release contains, among other things, a substantial simplification and
cleanup of the loader, which should fix a number of issues and also make it
forward compatible with extensions newer than the loader itself. As a part of
this change, the loader itself now only supports a single XrInstance active at
a time per process. If you attempt to create a new instance while an existing
one remains (such as in the case of application code leaking an XrInstance
handle), the loader will now return XR_ERROR_LIMIT_REACHED.

Windows binaries of the loader, built with Visual Studio 2019, are attached.