Let the input bufferize more data when possible.

Let the input bufferize more data when possible.

 The input will try to read 20% faster the source until a certain amount
of data is buffered by the fifo of the decoders (for now 10Mbytes or
500kbytes when OPTIMIZE_MEMORY is defined for the sum of all fifos).
 This buffering adds up to pts_delay without any additional delay but can
only work when VLC controls the source pace.

 It has a drawback with the current way the meta data works: they are seen
too soon.