Iterator broadcast persistence can break Fill Alpha

[theflyingzamboni]

When you use an iterator, Load Image (Iterator) only updates the image info broadcast when you run the chain. Since it can't know image info ahead of time, the validity checker does not catch if you have an RGB image connected to Fill Alpha, which requires an RGBA image. It only errors when you attempt to run the chain and the channel assertion in Fill Alpha fails. The issue here is that once Load Image (Iterator) has been run, the broadcast for the last loaded image persists. So if you run a folder with an RGB image and it fails on Fill Alpha, the load node will continue to output RGB as the type, which causes Fill Alpha to register as invalid. Since it's invalid, you can't run the chain even if the directory is changed and all images in it are valid RGBA images.

To test:

1. Create a simple iterator with Fill Alpha connected to Load Image
2. Input a directory with RGB images and run
3. Change directory to anything, the chain will not run