segment: Change back gst_segment_do_seek() to use gint64

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#794215)

Description

During 0.11 dev at commit bdbc0693 the gst_segment_do_seek() was changed to accept only unsigned start/stop. Though, the code will do direct comparision with -1 and also assumes negative values for GST_SEEK_TYPE_END:

start = segment->duration + start;

Additionally, the gst_event_new_seek() API along with the GstElement API uses signed integer. Demuxers will pass these value unchecked to that API in order to update their segment. Looking at the code, it will properly wrap around resulting in the same behaviour, but it's all a bit of a lie. As this does not affect the API, I think it's fine to switch it back to gint64.