RFR: 8268219: Investigate gstmpegaudioparse PTS issue

[Alexander Matveev]

- Reverted JDK-8268152 fix in gstbaseparse.c, since it is no longer needed.
 - Our hlsprogressbuffer outputs buffers in time format, but without any PTS. After GStreamer update mpregparser no longer tries to figure out timestamps if stream in time format and it will assume that upstream provides timestamps. Fixed by providing starting timestamp at the beginning or after seek. In this case mpegparser able to figure out timestamps and will provide them for each buffer downstream.
 - Segment start was also incorrect it should be seek position, otherwise after seek playback waits for seek time. For example if we seek to 2 min, mediaplayer hangs for 2 min and only after that resumes playback. I think it worked before, since mpegparser handled PTS before.

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Commit messages:
 - 8268219: Investigate gstmpegaudioparse PTS issue

Changes: https://git.openjdk.java.net/jfx/pull/532/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=532&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268219
  Stats: 19 lines in 2 files changed: 13 ins; 4 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/532.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/532/head:pull/532

PR: https://git.openjdk.java.net/jfx/pull/532