pdfrenderer.cpp: Ignore non-text blocks

Fix tesseract-ocr#3957.

Author: amitdo

src/api/pdfrenderer.cpp

@@ -354,6 +354,10 @@ char *TessPDFRenderer::GetPDFTextObjects(TessBaseAPI *api, double width, double
   const std::unique_ptr</*non-const*/ ResultIterator> res_it(api->GetIterator());
   while (!res_it->Empty(RIL_BLOCK)) {
     if (res_it->IsAtBeginningOf(RIL_BLOCK)) {
+      auto block type = res_it->BlockType();
+      if (!res_it->PTIsTextType(block_type)) {
+        continue; // ignore non-text blocks
+      }
       pdf_str << "BT\n3 Tr"; // Begin text object, use invisible ink
       old_fontsize = 0;      // Every block will declare its fontsize
       new_block = true;      // Every block will declare its affine matrix