Fix: Possible double free when resetting PGS subtitles.

This resulted in random crashes when seeking quickly in a file with PGS subtitles and other various cases.

Fixes #4240.
Update translations from Transifex:

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Fixes #4268.
Don't prevent the users from using the ISR if they choose not to block VSFilter like filters.

This commit fixes ticket #4231.
PGS subtitles: Enqueue empty segments to be sure subtitles are properly cleared when using the queue.

This is a work-around and will be improved later.

Refs: #3691.
Update translations from Transifex:

 - Croatian
 - Dutch
 - German
 - Japanese
 - Spanish
 - Ukrainian
 - Vietnamese
Update files for v1.7.5 release.
Update docs/Release.txt.
Set MPC_NIGHTLY_RELEASE to 1.
Readme.md: switch to SVG badge.