[cairo] Implementing cairo_read_func_t/cairo_write_func_t properly

[Lawrence D'Oliveiro]

On Wed, 30 Dec 2015 15:27:13 +0100, Bernhard Fischer wrote:

> when I would return STATUS_READ_ERROR, then the calling function
> cannot detect how many bytes then actually have been read except in
> the case that I read 1 byte after the other.

Does it matter? In any case, the file is clearly corrupted and the
image decoding has failed.

At least, that is my conclusion...