srtenc: Add timing-less "subrip" encoder.

Unsurprisingly, if a timing-less subrip decoder is desireable, an
encoder is as well. With this in place, we can move on to remove
the use of the old encoder/decoder with embedded timing and move
all timing handling the (de)muxer where they belong.

Signed-off-by: Philip Langdale <philipl@overt.org>

libavcodec/allcodecs.c

@@ -415,7 +415,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER (REALTEXT, realtext);
     REGISTER_DECODER (SAMI, sami);
     REGISTER_ENCDEC  (SRT, srt);
-    REGISTER_DECODER (SUBRIP, subrip);
+    REGISTER_ENCDEC  (SUBRIP, subrip);
     REGISTER_DECODER (SUBVIEWER, subviewer);
     REGISTER_ENCDEC  (XSUB, xsub);