The PNG parser must inform the user if the compression method is not supported

Today I was looking at the ICCP chunk type and the PNG image parser, and noticed that it was using the deflate compression only, regardless of the compression method value passed by the user.

The documentation does say that deflate (0) is the supported compression, but that vendor implementations may use different compression algorithms. So instead of trying to use deflate (which would raise a runtime exception), we should inform the user that the compression method is not supported.