Add alpha conversion helpers

[madsmtm]

Provide a few helper functions for handling the alpha channel when the pixel format of the buffer is PixelFormat::Bgra8 or PixelFormat::Rgba8.

I'm a bit unsure whether it makes sense to provide these? Their current implementations are quite simple, and if we can provide AlphaMode::Premultiplied everywhere, the [un-]premultiplication functions probably aren't really necessary. And perhaps the use-case of actually needing AlphaMode::Ignored is kinda niche, which makes make_pixels_opaque unnecessary too?

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I think the biggest problem with providing these is that it kinda signals that they're fast implementations, which... They aren't really, they could be made much more performant using SIMD and/or rayon.

I know that I don't want Softbuffer to be in the business of providing drawing primitives like tiny-skia or vello_cpu does, and I guess you could argue that this is kinda in that category, at least in the sense that it's a very different set of things you need to worry about (CPU speed vs. abstracting platform details and managing buffers).