Premultiplied alpha

By convention MuPDF holds pixmaps in `premultipled alpha' form. This means that when an alpha plane is present, the values for the process and spot colors are stored scaled by the alpha value.

So for a pixel with R=G=B=1, with solid alpha, we'd have values of 255, but with an alpha value of 0.5 we'd have values of 127 stored.

This format is used because it simplifies many of the plotting and compositing operations used within MuPDF.