add unary minus for ImVec2

[koostosh]

Unary minus can be declared as non-member, so no reason to not expose it. Adds potentially missing functionality for ImVec2 to reduce usage of IM_VEC2_CLASS_EXTRA, especially that other math operators can be now easily esposed with IMGUI_DEFINE_MATH_OPERATORS.

[ocornut]

I’ll reinvestigate this because I don’t recall the exact reason for that comment.

But “to reduce usage of IM_VEC2_CLASS_EXTRA” seems like a misunderstanding. The point of this is that people prefer using their own types, so this system isn’t to avoid implementing functions on our end, it is to allow people to use their type and their function more naturally.

[koostosh]

I (and I guess many others) don't have 'own' vector class they really want or need to use to handle UI coordinates. If all basic math functionality can be provided by imgui then I don't have any reason to use IM_VEC2_CLASS_EXTRA for custom casts or anything else. That's why I think this will remove some usecases of this macro, and unless you want to force people to use their types instead ImVec2 I think it is an argument for this change.

[ocornut]

This makes senses. It looks like it was my mistake/overlook to think this could only be overloaded as a member, it looks like it was never the case. Merged, thank you!

Recap https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Arithmetic_operators

[koostosh]

thanks man, keep up the good work!