osd: customizable subtitle and OSD position

You can set in which "corner" the OSD and subtitles are shown. I'd
prefer it a bit more general (so you could set the alignment using
a factor), but the libass API does not provide this.

sub/osd.h

@@ -133,6 +133,8 @@ struct osd_style_opts {
     float spacing;
     int margin_x;
     int margin_y;
+    int align_x;
+    int align_y;
     float blur;
 };