lavu: add escape API

The escape API will be useful to perform escaping programmatically, which
is required when crafting argument strings, and will be used for context
printing as well.

This is based on the ffescape tool code, with a few extensions and fixes.

libavutil/avstring.c

@@ -28,6 +28,7 @@
 #include "common.h"
 #include "mem.h"
 #include "avstring.h"
+#include "bprint.h"
 
 int av_strstart(const char *str, const char *pfx, const char **ptr)
 {
@@ -267,6 +268,23 @@ const char *av_dirname(char *path)
     return path;
 }
 
+int av_escape(char **dst, const char *src, const char *special_chars,
+              enum AVEscapeMode mode, int flags)
+{
+    AVBPrint dstbuf;
+
+    av_bprint_init(&dstbuf, 1, AV_BPRINT_SIZE_UNLIMITED);
+    av_bprint_escape(&dstbuf, src, special_chars, mode, flags);
+
+    if (!av_bprint_is_complete(&dstbuf)) {
+        av_bprint_finalize(&dstbuf, NULL);
+        return AVERROR(ENOMEM);
+    } else {
+        av_bprint_finalize(&dstbuf, dst);
+        return dstbuf.len;
+    }
+}
+
 #ifdef TEST
 
 int main(void)