* Let the Gradient and Fill tools paint blank layers with whole-expanse gradients and colors
* Make it work on layers with un-fed Transform nodes
* Show the Fill tool's hover pattern over the whole expanse of layers painted by a color chain
* Let the Fill and Gradient tools replace one another's whole-expanse paint on a selected layer
* Keep a shared paint node alive when a tool takes over a layer, and share the paint value lookup
* Stop a chain node from being shifted twice when a node is inserted in front of it