Add alpha mask utilities to ImageBuffer

[197g]

The respective methods on DynamicImage are blocked on the type representation. We can't use any specific pixel type if the goal is preserving the type but DynamicImage has no Luma32F variant. For applying a mask we would have to take the parameter type as a non-DynamicImage somehow? Feels weird, we should just resolve to add at least 32-bit Luma to the crate, even if we do not add 32-bit LumaAlpha.

So I don't think this should close #1786 yet.