Locally "peeking through" the overlaid node graph to view the artwork underneath

A future feature for the node graph: hold Space to pan as well as display a bubble/magnifying glass sort of circle to show through the node graph fade to the underlying image without being darkened. Useful for viewing correct colors and stuff in a certain area, without adjusting the full  widget value or hiding and showing the graph.

Start by locating the node graph interaction entry point and the Space-key pan handling. Define how a bubble or magnifying circle reveals the underlying artwork without the graph fade, while preserving normal panning and the existing widget visibility behavior. Done means the localized artwork view works while holding Space and does not require changing the full widget value.

索引モデルが issue の本文から書いたものです。