osd: cleanup: make OSDTYPE_ constants private to OSD code

No need to have them everywhere. The only exception/annoyance is
MAX_OSD_PARTS, which is now basically duplicated (and at runtime
initialization is checked with an assert()).

sub/osd_state.h

@@ -7,6 +7,18 @@
 
 #define OSD_CONV_CACHE_MAX 4
 
+enum mp_osdtype {
+    OSDTYPE_SUB,
+    OSDTYPE_SUB2, // IDs must be numerically successive
+
+    OSDTYPE_OSD,
+
+    OSDTYPE_EXTERNAL,
+    OSDTYPE_EXTERNAL2,
+
+    OSDTYPE_COUNT
+};
+
 struct ass_state {
     struct mp_log *log;
     struct ass_track *track;