"get a copy of the image contents of a context" does not handle case where canvas is unconfigured #4606
Description
In get a copy of the image contents of a context, we have this line:
- Let alphaMode be context.[[configuration]].alphaMode.
This assumes there is a configuration. If the canvas is unconfigured (has never been configured or has been unconfigure()d), we need to specify what should happen. Almost certainly it should return a transparent image (which makes the canvas invisible, but also makes it provide a transparent image when used as an image source), though our default alphaMode is "opaque" so it could also go the other way.