Allow calling updateImage or draw whether an image has been loaded or not

Changes

This PR:

• removes the code which checks for a loaded image from the updateImage and draw methods.
• adds some tests to ensure drawImageSync is correctly verifying an elements state.
• fixes the condition in drawImageSync because of an edge case condition where the element has a layer added without an image.

Some context

I'm not sure if this was the original intention of updateImage but it is often used to tell cornerstone that the canvas state is invalid and needs to be re-rendered.

There are several similar methods drawImage, draw, invalidate, invalidateImageId and drawInvalidated.

• All these end up setting one or both the flags needsRedraw and invalid on one or more enabledElement's.
• None of these methods ever actually use any of the properties of the image.
• Only draw and updateImage currently have a check to see if the element has an image loaded.

This check is not needed

drawImageSync is the method which actually causes something to be rendered to the canvas and is therefore the method which would encounter an error if some required state were missing. The implementation of this method already contains checks to ensure the enabledElement has all the state required to render and will quietly return early if it does not.

So if these checks are working correctly, there is no need for draw or updateImage to also check this state.

Why is it worth changing this

The most obvious example of updateImage throwing causing pain is when a cornerstone tool is changing mode.

The tool may need to tell cornerstone that the state of the canvas needs to be updated. For example to stop showing a cursor, or to redraw without an annotation.

We can see a case of this causing a crashing bug here:
cornerstonejs/cornerstoneTools#1303

This has also been an issue with the BrushTool.