Add percent threshold to determine when to start buffering

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https://github.com/mpv-player/mpv/labels/meta%3Afeature-request

Note: https://github.com/mpv-player/mpv/pull/5787 a pull request very similar to my feature request

Expected behavior of the wanted feature

Add a feature to delay buffering until the remaining cache reach a percent of the demuxer-max-bytes.

This will be helpful for watching large videos offline (specially with an external hard drive) with sizes more than demuxer-max-bytes.

The current behavior is it starts buffering continuously to satisfy demuxer-max-bytes. It makes the external HDD spin non stop until the video is paused or closed. The OS can't put the ext HDD to sleep leading to worse battery life for laptops.

Alternative behavior of the wanted feature

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Start by reviewing pull request 5787 and the existing handling of demuxer-max-bytes. Determine how the remaining cache threshold should control buffering for large offline videos, and consider the battery-life behavior described in the issue. Done means the feature delays buffering until the configured percentage threshold is reached.