Attribute to texture node.

I'd like to be able to generate normal and bump maps from the point data. So an 'attribute to texture' node would be awesome. The node should have two modes:

Auto: Generates a new texture whenever the node tree is executed, provided the attribute hash is different from the last time the image was generated.

Manual: Generates when the user clicks a 'generate' button on the node. This button should also be exposable to the modifier's UI.

To make it so a single modifier button can trigger the generate button on multiple nodes, a new trigger group input type could be added, allowing multiple nodes to be wired to the same trigger input.