Added various missing operators to ImVec4

[gan74]

Added operator*(ImVec4, float), operator/(ImVec4, float), operator*(ImVec4, ImVec4) and operator-(ImVec4)

This makes ImVec4 (with IMGUI_DEFINE_MATH_OPERATORS) usable for basic math which is useful when manipulating colors.

I omitted self-assignment operators because they are easier to manage without.

[ocornut]

Hello,

Thanks for your PR.

The idea is to avoid turning the thing into a full-fledged math library. As far as I know, I don't consider those particular operator to be necessary or frequently used for color manipulation, e.g. why would you multiply all 4 fields of a color? or subtract two of them? Can you clarify why do you need them? (Genuinely curious and happy to be convinced)

There is a ImLerp() function in imgui_internal.h if that's what you are trying to implement, but otherwise note that you can perfectly add those operators in your own code.

[gan74]

The usecase is mostly lerp indeed!

The argument for having these as part of ImVec4 rather than relying on eg: ImLerp is that it allows ImVec4 to go through templated functions like any other vector type. I think there is a least surprise argument as well, I would expect a numeric vector type to either provide the 5 operators (+, -, *, / and unary -) or none at all.

All this can be done by adding the operators into imgui_user.h, but the change seemed simple enough that might be worth providing it by default.

[ocornut]

Thanks. Alright, I decided there's little harm in adding them, pushed as 7674cbc. Thanks!