segment: Make times signed and a fraction, and never return NONE for time conversions for out-of-segment values
Submitted by Sebastian Dröge
+++ This bug was initially created as a clone of Bug 756564 +++
+++ This bug was initially created as a clone of Bug 748316 +++
For 2.0, we should make the times signed and always return a value in those functions even if they are out of segment. gst_segment_to_running_time_full() already does this, but the same is needed in general for other situations too.
While at that, we should also reconsider making all times into a fraction.