Centered subtitles

Would anyone be interested on adding support for this? Right now subtitles are rendered the way it's displayed on the top of the image (being built from a reference point, to the top, with all lines of dialogues, single or double, being rendered above the reference point), but I believe there's no objection against them looking better on the bottom example (being rendered at the center of the reference point, and distributing one line of dialogue to the top, and one to the bottom, in case of double-line dialogues).

It looks more properly aligned. Rather than imposing double-line metric to single-lines, it recognizes the single-line as a different entity that requires different positioning.

And, yes, I also believe this would, too, look better on the cases in which the aspect ratio would force the user to position the subtitles on top of the picture due to lack of black space.

Use the reference image and issue description as the visual specification for centered subtitle placement, including single- and double-line dialogue. Start by locating mpv's subtitle-rendering entry point; done means subtitles match the requested alignment in both normal placement and cases where aspect ratio moves them above the picture.