pdf_render_page: Now support bg argument

[trevorld]

• Adds a bg argument to pdf_render_page() supporting "transparent" and all opaque paper colors defaulting to "white"
• Adds can_render_transparent_bg to poppler_config()
• To be explicit {grDevices} and {grid} were added to DESCRIPTION but since R CMD check doesn't warn about base packages these could alternatively be omitted

closes #141

[trevorld]

Also here is a Dockerfile that installs the poppler-cpp from Debian Unstable on top of Debian Testing which I used to test on Linux.

Dockerfile