subpictures: turn the "subtitle in black bars" into an opt-in feature

The b_subtitle flag doesn't really mean the subpicture is a subtitle, just that it has usable timings. The b_absolute flag is also only meaning it's absolute in video coordinates. We introduce a third flag b_in_window that tells whether the positioning is in video coordinates or in window coordinates. It can be absolute or relative (top/bottom, left/right alignment).

The b_in_window is off by default so nothing is displayed outside of the video (i.e. in black bars) by default. Each codec needs to be check what regions it can display outside of the video. The same subpicture can have regions positioned in the video and regions positioned in the window.

Both the video size and the window size are provided to the SPU updater.

The "subtitle in black bars" is enabled for text-only subtitles for now.

The OSD widgets are now displayed in black bars.

The USF subtitle codec can now handle positions relative to the window rather than just the video.