This fixes several issues with compatibility with C++20. C++23 was also
tested, but apparently it does not have any additional breaking changes
compared to C++20 (or we don't have those).
VCMI still uses C++17 as before - goal is only to make potential
transition easier.
There were 2 cases that are deprecated in C++20 that we use:
- Floating point operations on enums are deprecated
- `this` can no longer be captured when using default capture by value
`[=]`
Both of those should now be replaced with code that works fine in both C+
+17 and in C++20 mode
