[SubtitleSampleReader] Fix packet pts

[CastagnaIT]

Description

i suspect a regression of PR #1890
that solved the problem of wrong pts comparisons on CSession::GetNextSample
looks like that the use of PTSDiff is not needed anymore since
CSession::GetNextSample now use "manifest based" pts

i hope this dont cause sideeffects on other subs use cases, touch this code cause always headaches due to difficult in testing all cases

Motivation and context

@cscd98 reported privately that D+ has again subtitle problems
NOTE: D+ have use case of segmented webvtt, on multiple periods

i realized that the problem can be reproduced this way:
play video->enable subs from GUI->seek video back of some time->after the seek the subs segments are initially downloaded but not sent to kodi demuxer ofc subs are now broken

How has this been tested?

The only stream i found affected with this similar problem is
HLS "daserste ARD" tv live stream

and to test HLS multiperiods subs, i used my local test stream "Akmai"

D+ confirmed by cscd98

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly