We have a value to represent invalid/overflow anyway, so let's make sure
that all operations are actually not overflowing and if they do we
assigned that value to the result.
FormatValue is now renamed to GenericFormattedValue and the API slightly
changed. In addition there is now a FormattedValue trait, and a
SpecificFormattedValue trait plus types for Bytes, Buffers and the
existing ClockTime.
This allows to create functions like
Pad::query_duration<F: SpecificFormattedValue>() -> Option<F>
and doesn't require the caller to unwrap the generic value anymore,
which is completely unneeded in these cases.
In addition, Segment became FormattedSegment<T> with API to
upcast/downcast between the specific formatted values and the generic
formatted value. This greatly simplifies usage of Segments.
And also implement a FormatValue type that holds a value together with
its format to make usage of the positions/durations/seek positions/etc
more convenient and safe.
Fixes https://github.com/sdroege/gstreamer-rs/issues/3
