This commit makes the feature staging header build in OS razzle.
Retrieved from https://microsoft.visualstudio.com os.2020 OS official/rs_wdx_dxp_windev cd1428ce557238663b7a748763ea8ce082b88256
Related work items: MSFT-33722503
This change is necessary as the dep/ folder is not synced into the
Windows source tree.
I've also added a build rule producing a lib for {fmt}.
This will be required for our next OS ingestion.
## Summary of the Pull Request
Introduces a type that is basically an array (stack allocated, fixed size) that reports size based on how many elements are actually filled (from the front), iterates only the filled ones, and has some basic vector push/pop semantics.
## PR Checklist
* [x] I work here
* [x] I work here
* [x] I work here
* [ ] I'd love to roll this out to SomeViewports.... maybe in this commit or a follow on one.
* [ ] We need a TIL tests library and I should test this there.
## Detailed Description of the Pull Request / Additional comments
The original gist of this was used for `SomeViewports` which was a struct to hold between 0 and 4 viewports, based on how many were left after subtraction (since rectangle subtraction functions in Windows code simply fail for resultants that yield >=2 rectangle regions.)
I figured now that we're TIL-ifying useful common utility things that this would be best suited to a template because I'm certain there are other circumstances where we would like to iterate a partially filled array and want it to not auto-resize-up like a vector would.
## Validation Steps Performed
* [ ] TIL tests added
