Render filled form fields into Glyphs

Now form data will be rendered as usual along with the rest of the
content into the output.

This changes the page's Glyphs structure to include filled form field
characters and their bounding boxes.

I have tested this locally on a few PDFs. We should test it a little
more.

src/main.cpp

@@ -308,6 +308,17 @@ void dump_document_meta(const std::string filename, PDFDoc *doc,
 
 void TextPageDecRef(TextPage *text_page) { text_page->decRefCnt(); }
 
+void render_annotations(std::unique_ptr<Gfx> &gfx, Annots *annots) {
+  gfx->saveState();
+
+  for (auto i = 0; i < annots->getNumAnnots(); i++) {
+    auto *annot = annots->getAnnot(i);
+    annot->draw(gfx.get(), false);
+  }
+
+  gfx->restoreState();
+}
+
 typedef std::unique_ptr<TextPage, decltype(&TextPageDecRef)> TextPagePtr;
 
 TextPagePtr page_to_text_page(Page *page) {
@@ -319,7 +330,14 @@ TextPagePtr page_to_text_page(Page *page) {
                       -1, -1, -1, -1, false,           /* printing */
                       NULL, NULL));
 
+  gfx->saveState();
   page->display(gfx.get());
+  gfx->restoreState();
+
+  // Note: page->getAnnots() contains form fields, so this has the effect of
+  // rendering form fields.
+  render_annotations(gfx, page->getAnnots());
+
   dev->endPage();
 
   return TextPagePtr(dev->takeText(), TextPageDecRef);