MathUtil: C++20/23 modernisations

I've also created DegreesToRadians/RadiansToDegrees utility functions since there was a lot of repeated code

Remove the [[maybe_unused]] annotation from various parameters that are
unconditionally unused and comment out their names instead. This makes
it unambiguous that the variables are unused, while making the remaining
[[maybe_unused]] annotations more reliable indicators that those
variables are in fact used in some contexts.

These parameters are mostly in overridden functions where the override
doesn't need that particular variable.

Yellow squiggly lines begone!
Done automatically on .cpp files through `run-clang-tidy`, with manual corrections to the mistakes.
If an import is directly used, but is technically unnecessary since it's recursively imported by something else, it is *not* removed.
The tool doesn't touch .h files, so I did some of them by hand while fixing errors due to old recursive imports.
Not everything is removed, but the cleanup should be substantial enough.
Because this done on Linux, code that isn't used on it is mostly untouched.
(Hopefully no open PR is depending on these imports...)

VideoCommon: move global variables out of BPFunctions

VideoBackends / VideoCommon: add support for specifying include files in shader code