basesrc: re-negotiate caps in pull mode if necessary
Submitted by Matthieu Bouron
This patch solves the case when basesrc is stuck in pull mode and never get the chance to negotiate with downstream (to use a custom memory allocator for example).
It happens when typefind is plugged downstream, with the following scenario:
filesrc ! typefind ! demuxer (proposing a custom memory allocator).
Not sure if the patch is 100% correct though.