Removed temporary memory from the API

It was intended to make the API easier to use, but various automatic garbage collection all had flaws, and making the application periodically clean up temporary memory added cognitive load to using the API, and in many cases was it was difficult to restructure threaded code to handle this.

So, we're largely going back to the original system, where the API returns allocated results and you free them.

In addition, to solve the problems we originally wanted temporary memory for:
* Short strings with a finite count, like device names, get stored in a per-thread string pool.
* Events continue to use temporary memory internally, which is cleaned up on the next event processing cycle.

include/SDL3/SDL_properties.h

@@ -384,7 +384,12 @@ extern SDL_DECLSPEC void * SDLCALL SDL_GetPointerProperty(SDL_PropertiesID props
  * \returns the value of the property, or `default_value` if it is not set or
  *          not a string property.
  *
- * \threadsafety It is safe to call this function from any thread.
+ * \threadsafety It is safe to call this function from any thread, although
+ *               the data returned is not protected and could potentially be
+ *               freed if you call SDL_SetStringProperty() or
+ *               SDL_ClearProperty() on these properties from another thread.
+ *               If you need to avoid this, use SDL_LockProperties() and
+ *               SDL_UnlockProperties().
  *
  * \since This function is available since SDL 3.0.0.
  *
@@ -392,7 +397,7 @@ extern SDL_DECLSPEC void * SDLCALL SDL_GetPointerProperty(SDL_PropertiesID props
  * \sa SDL_HasProperty
  * \sa SDL_SetStringProperty
  */
-extern SDL_DECLSPEC_TEMP const char * SDLCALL SDL_GetStringProperty(SDL_PropertiesID props, const char *name, const char *default_value);
+extern SDL_DECLSPEC const char * SDLCALL SDL_GetStringProperty(SDL_PropertiesID props, const char *name, const char *default_value);
 
 /**
  * Get a number property from a group of properties.