Add a setter for `PageDrawer.graphics` member variable

Hey folks! The following is a request to add a `protected` setter in `PageDrawer` for the `graphics` member variable.

The idea behind this change is to be able to draw the entirety of a `TransparencyGroup` onto a separate image. We already tried using `showTransparencyGroupOnGraphics()` but this doesn't work well with deeply nested complex transparency groups (there are some areas that are drawn incorrectly, are missed or misaligned).

We set up a proof of concept that sets the parent graphics to the target image through reflection and it worked.

After the proof of concept we wondered if this could be included as a feature to avoid any usage of reflection.

We created a PR for this here: https://github.com/apache/pdfbox/pull/144

Thank you in advance guys.