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

include/SDL3/SDL_begin_code.h

@@ -67,6 +67,8 @@
 #  endif
 # endif
 #endif
+/* This is used to mark functions that return temporary memory */
+#define SDL_DECLSPEC_TEMP   SDL_DECLSPEC
 
 /* By default SDL uses the C calling convention */
 #ifndef SDLCALL