basetransform: Reconfigure before propose_allocation

basetransform: Reconfigure before propose_allocation

There exist cases where a reconfigure event was propagated from
downstream, but caps didn't change. In this case, we would
reconfigure only when the next buffer arrives. The problem is that
due to the allocation query being cached, the return query parameters
endup outdated.

In this patch we refactor the reconfigurating code into a function, and
along with reconfiguring when a new buffer comes in, we also reconfigure
when a query allocation arrives.

https://bugzilla.gnome.org/show_bug.cgi?id=753850