Proposal: indicate when software encoding is used

I think a lot of mpv users like me would be surprised to learn that it is not doing hardware encoding by default. Nothing is displayed when software encoding is used, and thousands of lines of man page have to be read to get down to --hwdec. So the user has to either wonder why CPU load is a little high, or stumble over a guide that mentions it.

While there are legitimate problems with enabling hwdec by default (as discussed in https://github.com/mpv-player/mpv/issues/12948), mpv could guide the many users for whom hwdec would be a good idea, to enable it.

I propose displaying something like this:

Not using hardware decoding. (Try --hwdec=yes to enable it or press Ctrl+h.)

In the same place that this is displayed when it is enabled:

Using hardware decoding (vaapi).

Start by locating the existing status message that reports “Using hardware decoding (vaapi)” and trace how hardware-decoding state is determined. Add the corresponding message for software decoding, including the suggested --hwdec=yes option or Ctrl+h shortcut, and verify that it appears in the same place.