added decoder_read()

On our way to stabilize the decoder API, we will one day remove the
input stream functions.  The most basic function, read() will be
provided by decoder_api.h with this patch.  It already contains a loop
(still with manual polling), error/eof handling and decoder command
checks.  This kind of code used to be duplicated in all decoder
plugins.

src/decoder_api.h

@@ -121,6 +121,15 @@ double decoder_seek_where(struct decoder * decoder);
 
 void decoder_seek_error(struct decoder * decoder);
 
+/**
+ * Blocking read from the input stream.  Returns the number of bytes
+ * read, or 0 if one of the following occurs: end of file; error;
+ * command (like SEEK or STOP).
+ */
+size_t decoder_read(struct decoder *decoder,
+		    InputStream *inStream,
+		    void *buffer, size_t length);
+
 /**
  * This function is called by the decoder plugin when it has
  * successfully decoded block of input data.