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.

include/SDL3/SDL_storage.h

@@ -52,10 +52,17 @@ extern "C" {
  * It is not usually necessary to do this; SDL provides standard
  * implementations for many things you might expect to do with an SDL_Storage.
  *
+ * This structure should be initialized using SDL_INIT_INTERFACE()
+ *
  * \since This struct is available since SDL 3.0.0.
+ *
+ * \sa SDL_INIT_INTERFACE
  */
 typedef struct SDL_StorageInterface
 {
+    /* The version of this interface */
+    Uint32 version;
+
     /* Called when the storage is closed */
     SDL_bool (SDLCALL *close)(void *userdata);
 
@@ -90,6 +97,15 @@ typedef struct SDL_StorageInterface
     Uint64 (SDLCALL *space_remaining)(void *userdata);
 } SDL_StorageInterface;
 
+/* Check the size of SDL_StorageInterface
+ *
+ * If this assert fails, either the compiler is padding to an unexpected size,
+ * or the interface has been updated and this should be updated to match and
+ * the code using this interface should be updated to handle the old version.
+ */
+SDL_COMPILE_TIME_ASSERT(SDL_StorageInterface_SIZE,
+    (sizeof(void *) == 4 && sizeof(SDL_StorageInterface) == 48) ||
+    (sizeof(void *) == 8 && sizeof(SDL_StorageInterface) == 96));
 /**
  * An abstract interface for filesystem access.
  *
@@ -176,8 +192,11 @@ extern SDL_DECLSPEC SDL_Storage * SDLCALL SDL_OpenFileStorage(const char *path);
  * should use the built-in implementations in SDL, like SDL_OpenTitleStorage()
  * or SDL_OpenUserStorage().
  *
- * \param iface the function table to be used by this container.
- * \param userdata the pointer that will be passed to the store interface.
+ * This function makes a copy of `iface` and the caller does not need to keep
+ * it around after this call.
+ *
+ * \param iface the interface that implements this storage, initialized using SDL_INIT_INTERFACE().
+ * \param userdata the pointer that will be passed to the interface functions.
  * \returns a storage container on success or NULL on failure; call
  *          SDL_GetError() for more information.
  *
@@ -186,6 +205,7 @@ extern SDL_DECLSPEC SDL_Storage * SDLCALL SDL_OpenFileStorage(const char *path);
  * \sa SDL_CloseStorage
  * \sa SDL_GetStorageFileSize
  * \sa SDL_GetStorageSpaceRemaining
+ * \sa SDL_INIT_INTERFACE
  * \sa SDL_ReadStorageFile
  * \sa SDL_StorageReady
  * \sa SDL_WriteStorageFile