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_stdinc.h

@@ -529,6 +529,49 @@ SDL_COMPILE_TIME_ASSERT(enum, sizeof(SDL_DUMMY_ENUM) == sizeof(int));
 extern "C" {
 #endif
 
+/**
+ * A macro to initialize an SDL interface.
+ *
+ * This macro will initialize an SDL interface structure and should be called before you fill out the fields with your implementation.
+ *
+ * You can use it like this:
+ *
+ * ```c
+ * SDL_IOStreamInterface iface;
+ *
+ * SDL_INIT_INTERFACE(&iface);
+ *
+ * // Fill in the interface function pointers with your implementation
+ * iface.seek = ...
+ *
+ * stream = SDL_OpenIO(&iface, NULL);
+ * ```
+ *
+ * If you are using designated initializers, you can use the size of the interface as the version, e.g.
+ *
+ * ```c
+ * SDL_IOStreamInterface iface = {
+ *     .version = sizeof(iface),
+ *     .seek = ...
+ * };
+ * stream = SDL_OpenIO(&iface, NULL);
+ * ```
+ *
+ * \threadsafety It is safe to call this macro from any thread.
+ *
+ * \since This macro is available since SDL 3.0.0.
+ *
+ * \sa SDL_IOStreamInterface
+ * \sa SDL_StorageInterface
+ * \sa SDL_VirtualJoystickDesc
+ */
+#define SDL_INIT_INTERFACE(iface)               \
+    do {                                        \
+        SDL_zerop(iface);                       \
+        (iface)->version = sizeof(*(iface));    \
+    } while (0)
+
+
 #ifndef SDL_DISABLE_ALLOCA
 #define SDL_stack_alloc(type, count)    (type*)alloca(sizeof(type)*(count))
 #define SDL_stack_free(data)