Also fix xbox ima decoder, it was wrong if the decode didn't always start at the block start. Note that this and the nds ima decoder both rely on being run through the file linearly.
I wasn't able to test this on mono wavm or xwav yet, but I think it ought to work.
git-svn-id: https://vgmstream.svn.sourceforge.net/svnroot/vgmstream@217 51a99a44-fe44-0410-b1ba-c3e57ba2b86b
