osd: simplify an aspect of change detection handling

There was a somewhat obscure optimization in the OSD and subtitle
rendering path: if only the position of the sub-images changed, and not
the actual image data, uploading of the image data could be skipped. In
theory, this could speed up things like scrolling subtitles.

But it turns out that even in the rare cases subtitles have such scrolls
or axis-aligned movement, modern libass rarely signals this kind of
change. Possibly this is because of sub-pixel handling and such, which
break this.

As such, it's a worthless optimization and just introduces additional
complexity and subtle bugs (especially in cases libass does the
opposite: incorrectly signaling a position change only, which happened
before). Remove this optimization, and rename bitmap_pos_id to
change_id.

sub/osd_state.h

@@ -36,8 +36,7 @@ struct osd_object {
     struct sub_bitmaps cached;
 
     // VO cache state
-    int vo_bitmap_id;
-    int vo_bitmap_pos_id;
+    int vo_change_id;
     struct mp_osd_res vo_res;
 
     // Internally used by osd_libass.c