Currently decodebin2 and multiqueue allow queues to grow up to 12Mb.
First they have a limit of 2Mbytes, and then if one queue is full and at least one other empty, that queue will be grown by an extra 10Mbytes.

This allows to handle formats with BIG interleaves (think HD streams), but the problem is that it is not very memory efficient for plenty of other cases, especially on embedded devices.

This should be fixed so that:
_ we use queues as small as possible, but big enough to allow proper de-interleaving
_ decodebin2 can handle the fact that some demuxers do not create all their pads at the beginning and, because of very big interleaving, we need to allow a single queue to grow enough to allow the other stream pads to appear.

Comments and suggestions are more than welcome.