Reads a chunk of bytes from the BLOB accessible through op into blob.
a #GdaBlob to read data to
offset to read from the start of the blob (starts at 0)
maximum number of bytes to read.
the number of bytes actually read. In case of error, -1 is returned and the provider should have added an error to the connection.
Reads a chunk of bytes from the BLOB accessible through op into blob.