IDBMirrorVFS.jTruncate() doesn't open a transaction

[rhashimoto]

As reported here by @subframe7536, a direct call to IDBMirrorVFS.jTruncate() without a preceding call to jWrite() fails because there is no active transaction. This isn't expected to be encountered when the VFS is called from SQLite because the updated page count will be written to page 0, but the file demo does make a call this way.

The proposed fix is to create an active transaction in jTruncate() as well as jWrite().

This bug is also present in OPFSPermutedVFS.