ref: add REF_D3D placeholder for compatibiltiy(not planned anyway)

engine/ref_api.h

@@ -149,10 +149,13 @@ enum ref_defaultsprite_e
 	REF_CHROME_SPRITE // cl_sprite_shell
 };
 
+// the order of first three is important!
+// so you can use this value in IEngineStudio.StudioIsHardware
 enum ref_graphic_apis_e
 {
 	REF_SOFTWARE,	// hypothetical: just make a surface to draw on, in software
-	REF_GL,			// create GL context
+	REF_GL,		// create GL context
+	REF_D3D,	// Direct3D
 };
 
 typedef enum