The documentation for the function ImageOptions() doesn't specify the units used for x, y, w, or h. What are they?
The documentation for the function [`ImageOptions()`](https://github.com/go-pdf/fpdf/blob/504c6dd8cc916cd7f2097877efd52cae5f1d8b18/fpdf.go#L3357) doesn't specify the units used for x, y, w, or h. What are they?