[CS2] Fix compilation errors for std=c++2a

[mixern6]

The PR fixes these compilation errors:

• error: increment of object of volatile-qualified type 'volatile int' is deprecated
• error: ISO C++17 does not allow 'register' storage class specifier
• error: too many arguments provided to function-like macro invocation clamp

Explanation:

• Incrementing of a volatile type is deprecated but can be replaced with assignment of the incremented value.
• Register storage is not allowed, but it is just a hint and the keyword can be safely removed: https://stackoverflow.com/a/3207025/18259736
• The clamp macro is no longer required and can be excluded: https://www.mail-archive.com/pld-cvs-commit@lists.pld-linux.org/msg474109.html There's a clamp function defined in basetypes.h in hl2sdk codebase so it gets picked up.

[GAMMACASE]

Thanks