*decodebin: make it possible to disable max-size-*
Submitted by Mathieu Duponchelle
decodebin considers max-size-* == 0 means "automatic", and
previously didn't expose any value to mean "disable".
We now consider G_MAXUINT* to mean disable, and translate 0 to
G_MAXUINT* in uridecodebin, this in order not to break the API.
From the command-line, one can use -1.