[dvipdfmx] Possible extensions to color special support

[Shunsaku Hirata]

> After some consideration I am now thinking that having
> a different set of special commands (other than pdf:) is
> appropriate for multiple color stack implementation, like
>
>   \special{pdfcolorstackinit 1 page direct (0 g)}
>   \special{pdfcolorstack 1 push (1 0 0 rg)}
>   \special{pdfcolorstack 1 set    (0 0 1 rg)}
>   \special{pdfcolorstack 1 current}
>   \special{pdfcolorstack 1 pop}
>
> Here PDF string objects are used for specifying PDF
> literals.
>
> Although they are not only for "color" but as they are for
> PDFTeX compatibility, pdfcolorstack is used.
>
> I already finished implementing them and after some
> tests they would be available.

pdfcolorstack special as mentioned above was already
committed. Valid range of stack id is 0 to 255.
And also note that pdfcolorstackinit must be always
called before any of pdfcolorstack operations. Optional
keywords "page" and "direct" for pdfcolorstackinit are
the same as in PDFTeX.

Thanks,
Shunsaku Hirata