Added an OpenVR video driver (thanks @cnlohr!)

include/SDL3/SDL_hints.h

@@ -2629,6 +2629,17 @@ extern "C" {
  */
 #define SDL_HINT_OPENGL_ES_DRIVER "SDL_OPENGL_ES_DRIVER"
 
+/**
+ *  Mechanism to specify openvr_api library location
+ *
+ *  By default, when using the OpenVR driver, it will search for the API
+ *  library in the current folder.  But, if you wish to use a system API
+ *  you can specify that by using this hint.  This should be the full or
+ *  relative path to a .dll on Windows or .so on Linux.
+ *
+ */
+#define SDL_HINT_OPENVR_LIBRARY              "SDL_OPENVR_LIBRARY"
+
 /**
  * A variable controlling which orientations are allowed on iOS/Android.
  *