OpenSystemsDevelopment / qpdf

Simplified code by removing redundant `this` qualifiers where they are not essential, improving readability and alignment with coding standards. No changes were made to functionality.

Increase to POINTERHOLDER_TRANSITION=4

Run this:

for i in  **/*.cc **/*.c **/*.h **/*.hh; do
  clang-format < $i >| $i.new && mv $i.new $i
done