Update save image function to export image without surrounding border 🐜

In Firebox the saveCanvas function saves a file with dimension of stacksize (width and height of the image), whereas in Chrome it seems to includes a black band around the outside, presumably canvassize which is currently defined as stacksize + canvasspace.

Update: Behaviour now appears consistent across browsers (perhaps issue identified above was a caching problem): both Firefox and Chrome save canvas based on canvassize (which is what you'd expect from the code). Ideally the save function should only export stacksize as per the following.

Current behaviour

When users click on "save image" button saveCanvas function exports canvassize which is currently defined as stacksize + canvasspace.

Desired behaviour

When users click on "save image" button they should ideally be able to export the area currently defined as stacksize, excluding the canvasspace area which has been included to allow space for save button, sliders, dropdown menus and other user interface features to customise and adjust the image.

This could possibly be addressed by splitting into two phases and/or screen areas:

1. A configuration, including inputting image URLs and adjusting settings;
2. Image canvas, for previewing image and showing what would be saved with the "save image" button.

Start by locating the saveCanvas function and the save image button, then trace how stacksize, canvassize, and canvasspace determine the exported region. Verify the current output dimensions and adjust the save behavior so it excludes the canvasspace area; confirm that the saved image contains only the stacksize region.