The previous fix (e62e73e4) only corrected text_rendering_mode →
text_mode in the single-page Fpdf2PdfRenderer, but the main OCR
pipeline uses Fpdf2MultiPageRenderer which still had the old
attribute name. Since fpdf2 has no text_rendering_mode property,
setting it silently created a no-op attribute while text_mode stayed
at FILL — so 3 Tr (invisible text) was never emitted.
Fixes#1631, #1632