FOP-2847: Support palette-based transparency via tRNS chunk of PNG im…

[7obywankenobi]

…ages in PDF export

java.awt.image.BufferedImage.getAlphRaster() return null if an
IndexColorModel is used. This resulted in those PNGs missing the
SMask and had broken transparency effects.

PNG images with indexed color palette featuring tRNS transparency
encoding had potentially wrong PDF Mask. If image contained transparency
but no fully transparent pixel, then the default implementation of
PDFImageXObject still generates a Mask entry with three color components.
PDF Specification expects the index when using a palette based color
model. Thus Acrobat Reader does not process this image and shows an
error message. In general ImageRenderedAdapter would overwrite this
Mask, but since
AbstractImageAdapter.getIndexOfFirstTransparentColorInPalette() does
return null if image does not contain fully transparent pixels the
more general implementation of PDFImageXObject leaks to the output.
This has been fixed.