Integrate Half-Life anniversary changes

[SamVanheer]

Integrate the changes made in the Half-Life anniversary update into this SDK. Normally this should be done by merging in the updated SDK that Valve announced. Ideally this should be done as follows:

1. Fork the newest version of the Half-Life 1 SDK
2. Make sure all files use UTF8 encoding (should hopefully be done by Valve already, but check first) and commit any encoding changes
3. add the .clang-format file from Half-Life Updated and format all files (script in the Unified SDK or a Visual Studio extension can do this among other tools). Needed to avoid massive amounts of merge conflicts caused by whitespace and newline changes
4. Commit formatted files
5. Remove any changes that do not meet the goals of Half-Life Updated (e.g. gameplay changes) or make them optional and commit
6. Add fork as remote to Half-Life Updated
7. Use git fetch to update remote info
8. Merge the commit that formatted files
9. Resolve merge conflicts; there will likely be many
10. Commit merge and push

In the event that the updated SDK is not made available the changes will need to be manually implemented. Mostly this boils down to observing in-game behavior and emulating it. It's also possible to use reverse engineering tools to look at what the new code is doing but this can be difficult to read.