Minor shader update and fixes

Upon actually implementing this shader into my game, I realized that my UI was rendering on both the PrimaryCamera as well as the camera which renders to the window, so I made it so only the PrimaryCamera renders UI now. I plan to make this customizable in the future incase you do want UI to render over the post process instead of under it.

I also added some basic scanlines to the shader which I think really goes a long way in improving the aesthetic. Thanks to sixfold from the Bevy Discord for this suggestion, I may implement more of their suggestions down the line as well.

First release

The crate has been published and should be fully functional for anyone wishing to use it. Future updates may include improvements to readability, usability, and performance. Provided below is a release containing the binary from the main.rs file which is just an example. There is also an assets.zip file with an assets folder for you to combine with your own assets folder if you're using this crate in your project.