Re-added SDL_SoftStretch() to the API

This is needed for sdl2-compat, since a blit is not quite equivalent.

src/dynapi/SDL_dynapi.sym

@@ -1233,6 +1233,7 @@ SDL3_0.0.0 {
     SDL_AudioStreamDevicePaused;
     SDL_ClickTrayEntry;
     SDL_UpdateTrays;
+    SDL_SoftStretch;
     # extra symbols go here (don't modify this line)
   local: *;
 };