BMP: Use NonZeroU16 to represent size internally#3046
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The BMP decoder already (poorly) enforced a limit of 65535 and rejected empty images. This PR makes this behavior explicit by representing width and height using
NonZeroU16. (I said "poorly" becauseParsedInfoHeader::parsedidn't enforce the limit correctly. See if you can spot the issue.)This makes local reasoning for overflow and underflow a lot easier. Now it's trivial that
height - 1won't underflow. Similarly, many calculations can now be trivially proven to never overflow by looking at types.This PR is a draft, because it doesn't handle ICO's
height /= 2correctly. I need #3042 for that.