[poppler] color inversion for a mask image

[Ihar `Philips` Filipau]

Hi All!

I have encountered minor yet interesting problem. With the
functionality I have recently added to pdftohtml, now also the mask
images are extracted (as PNGs).

In one document I have, one mask image is extracted with colors
inverted: white is black, black is white. Yet, it is displayed by all
the PDF viewers correctly.

Is there any flag/etc in GfxState/etc which might be responsible for
color inversion? the flag pdftohtml doesn't check yet?

Thanks.