Configure background mode

I'm considering having an alignment tool for background mode. Building this tool would help me work through the viewport alignment issue that I have now wherein scrolling doesn't follow the mouse cursor directly.

If I create a manual alignment tool for changing the background mode position, then the background world coordinates will be converted from hard coded values, specific to this image, to dynamic values that can be configured via text fields and interactive controls.

The two most likely methods for implementing this are:

• display a new points layer with picking for the four corners of the background image.
• allow the edges of the background image to be click/dragged.

The first method will allow for non-rectangular backgrounds, which is more desirable than clicking/dragging the background edge images. I'll investigate adding a new layer of control points and then computing their screen/world correspondence. This will make adjusting the background image relatively easy.

Start by tracing the existing background mode and viewport alignment behavior described in the issue. Investigate how background world coordinates are currently defined, then evaluate the proposed control-point layer and screen/world correspondence approach. Done means background positioning can be configured interactively and no longer depends on image-specific hard-coded values.