Make SourceObject return number of bytes decoded

This is in sync with SourceBlock(sbn).

pkg/raptorq/interface.go

@@ -134,7 +134,7 @@ type Decoder interface {
  // SourceObject copies the source object into the given buffer. buf should
  // contain enough space to store the entire source object (use
  // TransferLength() to get the required size).
- SourceObject(buf []byte) error
+ SourceObject(buf []byte) (n int, err error)
 
  // Free, on supported implementations, will free memory used for generating
  // encoding symbols for the given source block. Once a source block has