OCR - Expose document writer with OCR capabilities

[yveszoundi]

Rationale

In one of my apps, I started replacing few libraries with MuPDF and I noticed that MuPDF has Tesseract support (duplicate symbol errors with Leptonica in my app, only on Windows).

• My app uses leptonica-sys with tesseract-sys, I was able to "resolve" linkage errors with RUSTFLAGS (No issue on Linux and Mac OS).
• This led me to check if I can just use Tesseract with MuPDF and compile with crt-static flags (without additional flags such as -C link-arg=/FORCE:MULTIPLE)

Goal

I believe that "most" people don't have to use tesseract-rs or its related sys crates containing more advanced features. With mupdf-rs I can drop 5 dependencies in one of my apps (poppler-rs, cairo-rs, lopdf, tesseract-sys, leptonica-sys).

Proposal

I can send a pull request for the following:

• Expose a new method in mupdf-rs (DocumentWriter) to allow OCR via fz_new_pdfocr_writer
• This allows trivial branches for applications that optionally perform OCR, because it doesn't require a different data structure.
• I suppose that this would need to be guarded by the tesseract feature in the code

Other notes

I did some quick tests under Windows 11 and it's fine. I'll test soon under Linux and Mac OS prior submitting a pull request.

[yveszoundi]

I need a little bit of help, as I'm unable to fix Windows CI issues from GitHub Actions (MSYS2, arguments list too long).

Mac OS

The "neon" CPU flags were creating issues and I disabled them for now, not sure if there's an alternative to only enable them on Apple Silicon with cfg declarations. This makes tests pass on GitHub Actions.

Windows

In MSYS2 tests, I'm hitting CI issues mentioning that "the argument list is too long". I'm not familiar with C/C++ and Windows to an extent anymore, but Copilot suggests using a response file and I don't think can be enabled in mupdf-rs itself.

In the build.rs file, make is invoked using Command::new.

1 warning generated.
  5 warnings generated.
  thirdparty/tesseract/src/wordrec/tface.cpp:39:76: warning: unused parameter 'init_classifier' [-Wunused-parameter]
     39 | void Wordrec::program_editup(const std::string &textbase, TessdataManager *init_classifier,
        |                                                                            ^
  thirdparty/tesseract/src/wordrec/tface.cpp:40:47: warning: unused parameter 'init_dict' [-Wunused-parameter]
     40 |                              TessdataManager *init_dict) {
        |                                               ^
  thirdparty/tesseract/src/wordrec/tface.cpp:73:40: warning: unused parameter 'elapsed_time' [-Wunused-parameter]
     73 | void Wordrec::program_editdown(int32_t elapsed_time) {
        |                                        ^
  3 warnings generated.
  /bin/sh: line 1: /clang64/bin/ar: Argument list too long
  make: *** [Makefile:99: D:/a/mupdf-rs/mupdf-rs/target/debug/build/mupdf-sys-d607a7d30d364096/out/build/libmupdf-third.a] Error 126
  make: *** Waiting for unfinished jobs....
  thread 'main' panicked at mupdf-sys\build.rs:236:9:
  Build error, exit code 2
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace