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.