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.

src/storage/SDL_storage.c

@@ -153,6 +153,11 @@ SDL_Storage *SDL_OpenStorage(const SDL_StorageInterface *iface, void *userdata)
         SDL_InvalidParamError("iface");
         return NULL;
     }
+    if (iface->version < sizeof(*iface)) {
+        // Update this to handle older versions of this interface
+        SDL_SetError("Invalid interface, should be initialized with SDL_INIT_INTERFACE()");
+        return NULL;
+    }
 
     storage = (SDL_Storage *)SDL_calloc(1, sizeof(*storage));
     if (storage) {

src/storage/generic/SDL_genericstorage.c

@@ -182,6 +182,7 @@ static Uint64 GENERIC_GetStorageSpaceRemaining(void *userdata)
 }
 
 static const SDL_StorageInterface GENERIC_title_iface = {
+    sizeof(SDL_StorageInterface),
     GENERIC_CloseStorage,
     NULL,   // ready
     GENERIC_EnumerateStorageDirectory,
@@ -224,6 +225,7 @@ TitleStorageBootStrap GENERIC_titlebootstrap = {
 };
 
 static const SDL_StorageInterface GENERIC_user_iface = {
+    sizeof(SDL_StorageInterface),
     GENERIC_CloseStorage,
     NULL,   // ready
     GENERIC_EnumerateStorageDirectory,
@@ -259,6 +261,7 @@ UserStorageBootStrap GENERIC_userbootstrap = {
 };
 
 static const SDL_StorageInterface GENERIC_file_iface = {
+    sizeof(SDL_StorageInterface),
     GENERIC_CloseStorage,
     NULL,   // ready
     GENERIC_EnumerateStorageDirectory,

src/storage/steam/SDL_steamstorage.c

@@ -129,6 +129,7 @@ static Uint64 STEAM_GetStorageSpaceRemaining(void *userdata)
 }
 
 static const SDL_StorageInterface STEAM_user_iface = {
+    sizeof(SDL_StorageInterface),
     STEAM_CloseStorage,
     STEAM_StorageReady,
     NULL,   // enumerate