Support glow effect on a node

[Nithos]

To support another mode of selection identification want to support a glow effect on a per node basis.

1. Gradient colour offset below the main node
2. support for the custom node types as well
3. Shader?

[Nithos]

Not trivial
Currently everything is rendered to a single canvas on a per layer basis (no back buffer). Will need to add a backbuffer to be added.