[png-mng-implement] png_set_shift with palette images

[Jason S. <ja...@po...>]

If, when reading a palette image with libpng 1.5.9, I call 
png_set_shift() and then png_set_palette_to_rgb(), the bits get shifted 
twice as far as I asked for. (If I call png_set_shift *after* 
png_set_palette_to_rgb, then I get the correct result.)

For example, a white palette color (255,255,255) with significant bits 
(8,7,6) ought to be transformed to (255,127,63). But it actually gets 
transformed to (255,63,15).

There are probably also other ways to cause the bits of palette images 
to be shifted multiple times. Assuming it's a bug, I suggest the 
following fix:

  In pngrtran.c, inside the clause that begins:
    if ((png_ptr->transformations & PNG_SHIFT) &&
        (color_type == PNG_COLOR_TYPE_PALETTE)))

  add the line:
    png_ptr->transformations &= ~PNG_SHIFT;

-- 
Jason Summers