PDF support (import only; export coming soon!)

[tannerhelland]

(Relates to #541)

PhotoDemon can now load PDF files. Each page in the PDF is loaded as its own layer.

A variety of import settings are customizable at import-time, including...

• Document size and resolution
• Which pages to import (all pages, first page only, custom page ranges)
• Import pages with full transparency, or auto-composited against white (or any other background color)
• Various other rendering options, including control over antialiasing, annotations, and more

pdfium provides the back-end for rendering PDFs into editable raster layers. It now ships with PD by default.

I hope to add PDF export capabilities soon!