Yahoo! has released an update to its official Flickr app adding support for iOS 4 multitasking and several other new features. Flickr 1.2 now supports background task completion allowing in-progress photo uploads to complete after closing the app. The update now also provides support for posting uploaded photos to Twitter using a flic.kr short URL and uploading HD video. Other new features include iOS 4 Fast App Switching, enhanced full-screen browsing…