Introduce formal policy for APIs that return strings.

This declares that any `const char *` returned from SDL is owned by SDL, and
promises to be valid _at least_ until the next time the event queue runs, or
SDL_Quit() is called, even if the thing that owns the string gets destroyed
or changed before then.

This is noted in the headers as "the SDL_GetStringRule", so this will both be
greppable to find a detailed explaination in docs/README-strings.md and
wikiheaders will automatically turn it into a link we can point at the
appropriate documentation.

Fixes #9902.

(and several FIXMEs, both known and yet-undocumented.)

src/SDL_internal.h

@@ -293,6 +293,13 @@ extern int SDLCALL SDL_WaitSemaphoreTimeoutNS(SDL_Semaphore *sem, Sint64 timeout
 extern int SDLCALL SDL_WaitConditionTimeoutNS(SDL_Condition *cond, SDL_Mutex *mutex, Sint64 timeoutNS);
 extern SDL_bool SDLCALL SDL_WaitEventTimeoutNS(SDL_Event *event, Sint64 timeoutNS);
 
+/* Queue `memory` to be passed to SDL_free once the event queue is emptied.
+   this manages the list of pointers to SDL_AllocateEventMemory, but you
+   can use it to queue pointers from other subsystems that can die at any
+   moment but definitely need to live long enough for the app to copy them
+   if they happened to query them in their last moments. */
+extern void *SDL_FreeLater(void *memory);
+
 /* Ends C function definitions when using C++ */
 #ifdef __cplusplus
 }