ICO: Extract AND mask logic into function

[RunDevelopment]

Tiny bit of refactoring. I extracted the logic for reading and applying the AND mask out into its own function.

I also made 2 mindor changes:

1. I removed the height check. rgba.len() is always w*h anyway.
2. In the inner loop, I changed x >= width to x >= row.len(). These are equivalent, but the latter should help the compiler to optimize away bounds checks.