Set aout_buffer_t.b_discontinuity in mpeg audio packetizer.
Silent first 3 frames on discontinuity in mad decoder.
(close #590)

field to "attachment://NAME" where NAME is an attachment filename
(returned by DEMUX_GET_ATTACHMENTS)
- Lock input_item_t every time we access p_meta fields (there is still a problem
as playlist code does not always do so...).
- Do not preparse item we have already played.
- Wait that the current playing item is meta parsed before fetching its cover.

Added decoder_GetInputAttachments.

chromas converstion: optimize MMX/SSE acceleration even more by leveraging out of order execution, basically avoid CPU stalls by using more registers and by spacing memory fetch and use as much as possible (CPU will run available instructions while waiting for memory fecth to complete)