Refactor demo API test initialisation order

* Instead of only doing a very lightweight check to see if the API is available
  up-front, we now share the API initialisation among all API tests far enough
  to determine availability of extensions, features, etc. Then we can precisely
  determine which tests are available and which aren't before running.

util/test/demos/gl/gl_structured_buffer_nested.cpp

@@ -130,10 +130,10 @@ void main()
 
 )EOSHADER";
 
-  int main(int argc, char **argv)
+  int main()
   {
     // initialise, create window, create context, etc
-    if(!Init(argc, argv))
+    if(!Init())
       return 3;
 
     GLuint vao = MakeVAO();