Log Message:
Update dvipng to 1.12.

Changes:

        * Special.c: Keep transparent background in rescaled included bitmaps
        * special.c: Add the color PostScript prologue
        * dvipng.h, special.c: Support xcolor PostScript prologue
        * color.c: Support x11nam.def, fix handling of xcolor
          multiple-model color values, and xcolor PostScript prologue
        * special.c: Some (most?) literal PostScript specials seem to
          depend on tex.pro and possibly special.pro, always load these
        * configure.ac: Fix gs checks
        * misc.c: Correct last mmap element
        * readme.texi: Mention new color models in xcolor
        * color.c: Adjust for new version of xcolor, mainly color prefixes