Add OpenVR SDK to third_party

third_party/openvr/src/src/README

Index: third_party/openvr/src/src/README
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+This is the source code for the OpenVR API client binding library which connects
+OpenVR applications to the SteamVR runtime, taking into account the version
+of the OpenVR interface they were compiled against.
+
+The client binding library - openvr_api.dll on Windows, openvr_api.so on
+Linux, and openvr_api.dylib on macOS - knows how to find and read the
+SteamVR runtime installation information which allows it to find and
+dynamically connect to the installed runtime. In combination with the
+interface version identifiers from /include/openvr.h which are baked
+into applications at the time they are built, the OpenVR API client
+binding library captures and conveys to the SteamVR runtime the version
+of the OpenVR API interface behavior that the application expects.
+
+Applications carry with them a private/local copy of the client binding
+library when they ship, and they should install it locally to their
+application.  Applications should not install the client binding library
+globally or attempt to link to a globally installed client binding library.
+Doing so negates at least part of the ability for the client binding library
+to accurately reflect the version of the OpenVR API that the application
+was built against, and so hinders compatibility support in the face of
+API changes.
+
+Most applications should simply link to and redistribute with their application
+the pre-built client binding library found in the /bin directory of this
+repository. Some small number of applications which have specific requirements
+around redistributing only binaries they build themselves should build
+the client library from this source and either statically link it into
+their application or redistribute the binary they build.
+
+This is a cmake project, to build it use the version of cmake appropriate
+for your platform. For example, to build on a POSIX system simply perform
+
+  cd src; mkdir _build; cd _build; cmake ..; make
+
+and you will end up with the static library /src/_build/libopenvr_api.a