1.9.20

1. Optimize memory usage
2. Fix some crashes in loading
3. Improve Modify handling for textures
   celestiamobile/Celestia@c7724f9
   celestiamobile/CelestiaLocalization@47eeed2
   celestiamobile/CelestiaCore@c46c842
   CelestiaProject/CelestiaContent@d156a8b