Image transparency for PDF output

Before, drawing an image with transparency would result in black, opaque pixels in the PDF file. This change, figured out together with Jürg Lehni, fixes this. At its core, switching from drawImage(img, int, int, int, int, int, int, int, int, null) to drawImage(img, int, int, null) seems to do the trick - so this might have been a bug in iText all along.

Signed-off-by: Jürg Lehni <juerg@scratchdisk.com>
Signed-off-by: Gottfried Haider <gottfried.haider@gmail.com>
Add support for custom source regions in imageImpl

As suggested by Ben
Merge pull request #2070 from gohai/pdf-transparency

Image transparency for PDF output