Keep Externals/OpenXR pinned to Khronos' upstream 1.1.63 commit
instead of carrying local source edits as an unreachable submodule commit.
Record the three Frame loader compatibility changes in the parent tree as
OpenXR-Loader-Exceptions.patch.
When ENABLE_VR is configured, copy the pinned SDK into the binary tree,
apply the patch to that copy, and build the loader from there. This leaves
the checked-out submodule clean and supports read-only source mounts in
container builds. CMake verifies the patch applies to the expected SDK
files before adding the patched copy to the build.
Jeremy Bowers
18e1811dec