replace vrhr::state checking with nicer functions

sky.cpp

@@ -374,9 +374,7 @@ EX struct renderbuffer *airbuf;
 
 EX void make_air() {
   if(!sky) return;
-  #if CAP_VR
-  if(vrhr::state) return;
-  #endif
+  if(vrhr::active()) return;
   const int AIR_TEXTURE = 512;
   if(!airbuf) {
     airbuf = new renderbuffer(AIR_TEXTURE, AIR_TEXTURE, true);