basetextoverlay: Don't miscalculate text running times

basetextoverlay: Don't miscalculate text running times

When a new segment event arrives, it immediately updates
the current stored segment, which was used for calculating
the running time of the current text buffer for every
passing video frame. This means a segment that arrives
after the text buffer might get used to (mis)calculate
the running times subsequently.

Instead, calculate and store the right running time
using the current segment when storing the buffer. Later
the stored segment can get freely updated.

This fixes the case where pieces of video and text streams
are seamlessly concatenated and fed through the text overlay.
Previously, it could lead to the current text buffer suddenly
have a massive running time and blocking all further input.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2802>