Improve the pdftoraster filter

Currently, pdftoraster uses the Poppler libraries which are unstable and change their function definition after updates due to which there are building and installation errors. When the pdftoraster filter was originally written, it used XPDF APIs of Poppler. However, those APIs were unsupported in the recent versions of Poppler and completely removed from Poppler's source code. This affected cups-filters repository since pdftoraster filter was now unable to build or be used by anyone in any operating system. This also affected cups-filters as a whole, because it was unable to build. Hence, it was very important to find a solution to this problem.

The most plausible solution to overcome this problem is to instead of directly using the unstable internal APIs we can use stable APIs and rewrite any functionality which isn't even in stable APIs.