<!-- Enter a brief description/summary of your PR here. What does it fix/what does it change/how was it tested (even manually, if necessary)? -->
## Summary of the Pull Request
- Adds two simple animated shaders to the pixel shaders sample folder
- Updates the readme in the pixel shaders sample folder to add a section explaining the animated shaders
- Modifies some comments in existing shader samples
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## PR Checklist
* [ ] Closes #xxx
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [ ] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [ ] Schema updated.
* [x] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #8994
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## Detailed Description of the Pull Request / Additional comments
The two shaders I wrote are not especially pretty or interesting, but they should hopefully serve as simple examples for anyone looking to do animated effects. One simply draws a line of inverted pixels that scrolls down the screen, and the other fades the background back and forth between two colors.
I've added a new section to the readme explaining how the shaders work to achieve animated effects.
I've also updated the comments on the existing shaders to clear up a couple of things:
1. Be more explicit that `Time` represents seconds since the shader loaded. Though obvious in hindsight, this was not clear to me when I was first learning/experimenting
2. Explain that `tex` ranges from 0,0 to 1,1. This is important because, when trying to port GLSL shaders from shadertoy, I at first assumed `fragCoord` and `tex` are the same thing but the former actually ranges from 0,0 to the resolution of the canvas, so some of the math doesn't work out if you just substitute it with `tex`.
Any and all feedback welcome.
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## Validation Steps Performed
I ran the shaders manually in a dev build of the Terminal. I auto-spellchecked and manually proofread my additions to the readme and verifed the markdown rendering on github.
7c42ed4cc3
