Implement ECMA-119 burn_source and writer thread handling.

At this time, nglibisofs is able to output an image!! Note that communication
between writer and read thread is done via a pipe. That will be replaced by a
ring buffer in the near future.

src/libisofs.h

@@ -10,6 +10,8 @@
 
 #include <sys/stat.h>
 
+struct burn_source;
+
 typedef struct Iso_Image IsoImage;
 
 typedef struct Iso_Node IsoNode;
@@ -153,6 +155,9 @@ typedef struct {
  */
 int iso_image_new(const char *name, IsoImage **image);
 
+int iso_image_create(IsoImage *image, Ecma119WriteOpts *opts,
+                     struct burn_source **burn_src);
+
 /**
  * Increments the reference counting of the given image.
  */