Adds image and mask features, fixes clippath

This PR:
- Adds image support (Example: /test/pdfs/tutorial.pdf - has both paintnlineImageXObject and paintImageXObject)
- Adds mask support( Example: http://stderr.org/doc/libitext-java-doc/www/examples/Chap0613.pdf)
- Fixes clip (Example: Can be seen in many PDFs. Some clipped elements were not visible due to not applying transform)

rename `imgDataToPng` to `convertImgDataToPng` and add function name:
var convertImgDataToPng = (function convertImgDataToPngClosure() {