[Fix] Restore behaviour on a subtitle change after commit 6937fd4181b60f3ae20d664d697ec9589603c36f.
Commit 6937fd4181b60f3ae20d664d697ec9589603c36f unified/abstracted the Close(*)Stream functions.
The method CDVDPlayerSubtitle::CloseStream(bool flush) used the parameter flush in an opposed way
than any other class, which use bWaitForBuffers as parameter. This commit aligns that method.
Merge pull request #5273 from ace20022/ticket_15371

[Fix] When changing subtitles the cached overlays must be cleared.