Add support for flip row order BMP

[Tsuk1ko]

Refer to https://github.com/image-size/image-size/pull/89

A BMP with flip row order can be made by Photoshop:

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One more thing I found these two functions were named incorrectly:

https://github.com/nodeca/probe-image-size/blob/158c262a7e58b8e3abd7573e2d854299146fcc37/lib/common.js#L55-L67

They actually read signed integers, not unsigned integers.