Don't automatically free temporary memory, let the application call SDL_FreeTemporaryMemory() when it's ready.

Also mark up all functions that return temporary memory with SDL_DECLSPEC_TEMP, to help people implementing language bindings.

Fixes https://github.com/libsdl-org/SDL/issues/10378

src/dynapi/SDL_dynapi.sym

@@ -130,6 +130,7 @@ SDL3_0.0.0 {
     SDL_FlushEvent;
     SDL_FlushEvents;
     SDL_FlushRenderer;
+    SDL_FreeTemporaryMemory;
     SDL_GDKSuspendComplete;
     SDL_GL_CreateContext;
     SDL_GL_DestroyContext;