It allows to decrease the number of input thread wakeup and to
improve its reactiveness at user commands in case of data starvation.

 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.

Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

 * p_sys and p_sys->p_scene can't be NULL after the end of Create
 * use calloc instead of malloc+memset
 * remove dead initializations
 * use strdup to copy a string (not asprintf)

Signed-off-by: Rémi Duraffort <ivoire@videolan.org>

 I don't think there is any good fixed value, I have tuned it for
50mbit/s video with a cache of 60s.

Becareful about possible regressions.