Give warning about RAP verification

[msg7086]

@Asd-g @realfinder
Bringing this up because earlier today I put lots of effort into debugging the performance issue until I see the bug report 2 weeks ago about the change. I think this is a pretty big change that is worth mentioning as the performance penalty is quite significant on certain type of files.

On my NAS server, a random m2ts file takes 11 seconds to index with verification off, but takes 160 seconds with verification on. I can imagine that the numbers would go further up when working on 4k or 8k videos.

Shall we at least add a warning message before showing the index progress, to warn user, in both on and off case? If it's on, warn user that it will be very slow, and if it's off, warn user that it may be inaccurate.

[Asd-g]

I guess this mention isn't enough because there was another similar report.

Another option is to add Note in the main README.md like this one.

Also, default verification can be turned off. I had doubts about this because, on the one hand, there is a potential for a significant speed penalty, but on the other hand, the trustworthiness of the index should be paramount.

[msg7086]

Yeah because in some case the computer that indexes the files doesn't necessarily have the ability to decode the video quickly, since indexing has been a lightweight operation from the beginning. So if I have a NAS that stores the files and makes the index, and another faster compute node does the decoding and encoding, then the problem rises.

I would prefer having this verfication off by default, and users can enable it on demand. (When decoding clean sources like DVD or bluray turning verification on doesn't make a difference on the output really.)

If that's less desirable, then at least emit a warning message as part of stdout.

[Asd-g]

I changed default value to false.