Delete unused nodes and sockets for better performance

Deleting unused nodes and sockets can improve shader compilation and playback FPS

[x] Channel linear start and end nodes when it's using non-color data
[x] Normal Process and blending nodes when only bump map is used (Make sure preview mode will still works)
[x] Static One and Zero value (need further investigation)
[x] Texture coordinate node on root tree when there's no layer using non-UV vector
[x] Final bump process node when there's no layer using normal channel at all
[x] Start normal filter when no input is connected to ucupaint normal channel (normal filter apparently cannot be deleted)
[x] Layer UV socket when the layer is only using vertex color or other non-UV related
[x] Unused group channel nodes and sockets
[x] Disabled layer should only contains necessary nodes and sockets
[x] Transition bump/ramp/AO nodes
[x] Overlay normal when there's no layer using normal mapping
[x] Spread alpha before height process node (I'm not sure if this can be deleted or not)

ucupumar / ucupaint