issue #4904: fix iOS Image::safeToFile crashes when save jpg file.
closed #4904: use new Xcode command line tools to compile i386 arch libjpeg.a file, and shrink the lib file.
Merge pull request #6327 from boyu0/bug4904_Image_safeToFile_crashes_ios

closed #4904: fix ios Image::saveToFile crashes when save jpg file.