Audit ImageData code

[nox]

Things such as let len = width * height * 4; without any explicit overflow checking scares me.

[hgluka]

Hey, is it ok if I try to do this one? I'd have to ask you a few clarifying questions, but I think I can handle it, even though I'm a beginner in Rust.

[hgluka]

I looked into it myself, but I was wondering if there was a particular reason that width and height are u32, but get_size returns Size2D<i32>?

[jdm]

@lucantrop Probably because the particular caller that needed the size worked best in those units.