osd, lua: manage multiple ASS overlays set with set_osd_ass() calls

Until now, there was only 1 global ASS overlay that could be set by all
scripts. This was often perceived as bug when multiple scripts tried to
set their own ASS overlay.

This was kind of hard to solve because the script could set its own ASS
PlayResX/Y, which makes it impossible to share a single ASS_Renderer for
multiple scripts. The OSC unfortunately makes use of this feature (and
unfortunately can't be fixed because it's a POS), so we're stuck with
this complication.

Implement the worst-case solution and fix this by creating separate ASS
track and renderer objects for each script that wants to set an ASS
overlay.

The z-order is decided by the order the scripts set their text first.
This is essentially random, unless you do it at script init, and you
pass scripts in a specific order. Script initialization is currently
serialized (as a feature), so the first loaded script gets lowest
Z-order.

The Lua script API interestingly remains the same. (And also will remain
undocumented, unsupported, and potentially volatile.)

sub/osd.h

@@ -163,8 +163,6 @@ struct osd_progbar_state {
 };
 void osd_set_progbar(struct osd_state *osd, struct osd_progbar_state *s);
 
-void osd_set_external(struct osd_state *osd, int res_x, int res_y, char *text);
-
 void osd_set_external2(struct osd_state *osd, struct sub_bitmaps *imgs);
 
 enum mp_osd_draw_flags {
@@ -203,6 +201,9 @@ void osd_object_get_bitmaps(struct osd_state *osd, struct osd_object *obj,
 void osd_init_backend(struct osd_state *osd);
 void osd_destroy_backend(struct osd_state *osd);
 
+void osd_set_external(struct osd_state *osd, void *id, int res_x, int res_y,
+                      char *text);
+
 // doesn't need locking
 void osd_get_function_sym(char *buffer, size_t buffer_size, int osd_function);
 extern const char *const osd_ass_0;