lavu/bprint: add av_bprint_get_buffer().

It is useful to use bprint to handle a growing buffer
used with another API.

libavutil/bprint.c

@@ -129,6 +129,15 @@ void av_bprint_chars(AVBPrint *buf, char c, unsigned n)
     av_bprint_grow(buf, n);
 }
 
+void av_bprint_get_buffer(AVBPrint *buf, unsigned size,
+                          unsigned char **mem, unsigned *actual_size)
+{
+    if (size > av_bprint_room(buf))
+        av_bprint_alloc(buf, size);
+    *actual_size = av_bprint_room(buf);
+    *mem = *actual_size ? buf->str + buf->len : NULL;
+}
+
 void av_bprint_clear(AVBPrint *buf)
 {
     if (buf->len) {

libavutil/bprint.h

@@ -125,6 +125,18 @@ void av_bprintf(AVBPrint *buf, const char *fmt, ...) av_printf_format(2, 3);
  */
 void av_bprint_chars(AVBPrint *buf, char c, unsigned n);
 
+/**
+ * Allocate bytes in the buffer for external use.
+ *
+ * @param[in]  buf          buffer structure
+ * @param[in]  size         required size
+ * @param[out] mem          pointer to the memory area
+ * @param[out] actual_size  size of the memory area after allocation;
+ *                          can be larger or smaller than size
+ */
+void av_bprint_get_buffer(AVBPrint *buf, unsigned size,
+                          unsigned char **mem, unsigned *actual_size);
+
 /**
  * Reset the string to "" but keep internal allocated data.
  */

libavutil/version.h

@@ -75,7 +75,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  52
-#define LIBAVUTIL_VERSION_MINOR   1
+#define LIBAVUTIL_VERSION_MINOR   2
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \