Subtitle position differs from FFmpeg

[Arcus92]

There are two different concepts for positioning in PGS. A composition defines a simple x, y coordinate to render, but also a window - a rectangle with x, y, width and height.

Currently this lib uses the window position, however FFmpeg uses the composition position.

I guess the composition position always lays in the window rect, but is just more precise. So I should change it to match the FFmpeg position and check how it works. I will still use the full window for clearing.

[Arcus92]

For more context: jellyfin/jellyfin-web#6273