Delta patches do not apply if source files that need to be diffed are not writeable

[zorgiepoo]

From discussion #2210, if files in the source app bundle are not writeable and we copy the app bundle and do bspatch() on one of the files, it will fail if the destination file is not writable. At first I thought this wasn't worth fixing but I think this is easier to fix than I originally thought. In this case if the file is not writeable we can enter a code path we already have for copying new permissions and using a new fresh file.