color: add the concept of readyness to image descriptions

Creating an image description might involve reading from file descriptors which can take a significant amount of time. The previous specification required image descriptions to fail after a roundtrip which would make it impossible to send any events while creating an image description in a compositor.

This commit changes the object to become ready only after receiving a new ready event. Using the object before it is ready is an erro just like using the object after receiving a failed event.

Signed-off-by: Sebastian Wick sebastian.wick@redhat.com