TTA inaccurate seek

First reported here: http://forum.videolan.org/viewtopic.php?f=2&t=77852

When trying to seek any .tta (true audio) file, for example, using "Jump to specific time", clicking the progress bar or via cue sheets, the resulting position might be off by 5-10%, depending on the file. I suspect it happens because modules/demux/tta.c tries to seek by compressed frame size, rather than by decoded time.

Tested on win32 with VLC 1.0.5, VLC 1.1.0-rc3 and VLC 1.2.0-git-20100615-1138.