Initialize interface structures so they can be extended in the future

We guarantee that we will only add to the end of these interfaces, and any new fields will be optional.

test/testautomation_iostream.c

@@ -437,7 +437,7 @@ static int iostrm_testAllocFree(void *arg)
     SDL_IOStreamInterface iface;
     SDL_IOStream *rw;
 
-    SDL_zero(iface);
+    SDL_INIT_INTERFACE(&iface);
     rw = SDL_OpenIO(&iface, NULL);
     SDLTest_AssertPass("Call to SDL_OpenIO() succeeded");
     SDLTest_AssertCheck(rw != NULL, "Validate result from SDL_OpenIO() is not NULL");