PNG transparency of color 0 not working

[jake]

The NoDataValue is not interpreted as a transparent color value when it is set to 0 in the PNGCreateCopy function. This applies to both gray scale and RGB images but not images with a palette.

If the image is of type RGB and the MetaItem NODATA_VALUES is set to "0 0 0" the transparency value is correctly written to the PNG but not if GDALRasterBand::SetNoDataValue function is used set the nodata value to 0 for the bands. If the image is of gray scale type the NODATA_VALUES is not used at all and GDALRasterBand::SetNoDataValue does not work if the nodata value is 0.

[jake]

Patch for pgndataset.cpp to fix the reported bug

[Even Rouault]

Thanks. I've applied in r18008 a small variation of your patch where I also test the bHaveNoData flag to avoid writing a transparency value when there is none in the source dataset