Improve QOL by including additional addition/mutliplication arithmetic operators for ImVec2

[ArashPartow]

No description provided.

[ocornut]

Hello Arash,

My stance is that ImVec2 x float is justified but addition/subtraction isn’t. It’s an extremely rare and odd operation to perform, and adding those create a wider surface for mistakes being made and not reported by compilers. I don’t therefore believe they are a good idea.

Not having float x imvec2 is more arbitrary and arguable but since imvec2 x float is rare enough it felt like a legitimate trade off to explicitly only support one.

[ArashPartow]

@ocornut I only added addition and multiplication. As subtraction would have a different meaning.

The idea was to allow for the following operations:

ImVec2 v1;
ImVec2 v2;

v1 = 1.234 + v2;
v1 = 1.234 * v2;

That being said, I'm happy to remove the addition operator from the PR. Let me know, otherwise feel free to close the PR.