avcodec/movtextenc, srtenc, webvttenc: Mark encoders as init-threadsafe

They all rely on ff_ass_split(), which does not have any static state.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/movtextenc.c

@@ -718,5 +718,5 @@ const AVCodec ff_movtext_encoder = {
     .init           = mov_text_encode_init,
     .encode_sub     = mov_text_encode_frame,
     .close          = mov_text_encode_close,
-    .caps_internal  = FF_CODEC_CAP_INIT_CLEANUP,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
 };

libavcodec/srtenc.c

@@ -25,6 +25,7 @@
 #include "libavutil/bprint.h"
 #include "ass_split.h"
 #include "ass.h"
+#include "internal.h"
 
 
 #define SRT_STACK_SIZE 64
@@ -299,6 +300,7 @@ const AVCodec ff_srt_encoder = {
     .init           = srt_encode_init,
     .encode_sub     = srt_encode_frame,
     .close          = srt_encode_close,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };
 #endif
 
@@ -312,6 +314,7 @@ const AVCodec ff_subrip_encoder = {
     .init           = srt_encode_init,
     .encode_sub     = srt_encode_frame,
     .close          = srt_encode_close,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };
 #endif
 
@@ -325,5 +328,6 @@ const AVCodec ff_text_encoder = {
     .init           = srt_encode_init,
     .encode_sub     = text_encode_frame,
     .close          = srt_encode_close,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };
 #endif

libavcodec/webvttenc.c

@@ -26,6 +26,7 @@
 #include "libavutil/bprint.h"
 #include "ass_split.h"
 #include "ass.h"
+#include "internal.h"
 
 #define WEBVTT_STACK_SIZE 64
 typedef struct {
@@ -219,4 +220,5 @@ const AVCodec ff_webvtt_encoder = {
     .init           = webvtt_encode_init,
     .encode_sub     = webvtt_encode_frame,
     .close          = webvtt_encode_close,
+    .caps_internal  = FF_CODEC_CAP_INIT_THREADSAFE,
 };