[bmp] Header reading/writing functions should probably use references rather than owned arrays

[Lokathor]

We're passing some pretty hefty sized arrays. If an inline fails to happen, we'll be moving a lot of data across the function bound for no big reason.

Then again, how often do you open a bitmap? And assuming that you're doing a heap allocation too for the pixel buffer, maybe it's not a big deal.

Still, probably something to do.