SumatraPDF 3.1 release

• 64bit builds
• all documents are restored at startup if a window with multiple tabs is closed (or if closing happened through File -> Exit); this can be disabled through the RestoreSession advanced setting
• printing happens (again) always as image which leads to more reliable results at the cost of requiring more printer memory; the "Print as Image" advanced printing option has been removed
• scrolling with touchpad (e.g. on Surface Pro) now works
• many crash and other bug fixes