Update to synctex 1.19

Summary:
This should prevent crashes when reloading some synctex-enabled pdf
files created with newer versions of TeXLive. We also gain bugfixes,
features and improved accuracy from the last 6 years of synctex
development.

Procedure followed:
- svn co svn://tug.org/texlive/trunk/Build/source/texk/web2c/synctexdir
- Check out revision 45150
- Update files present in core/synctex/*
- Adapt Okular code to changes
- Review and drop or update/apply old patches using quilt
- Create missing patches for local synctex changes
- New patch: Omit warning message when opening non-synctex pdf
- Two new patches to fix more compiler warnings
- New patch: Plug multiple leaks and prevent a segfault

TODO for later:
- Move sync file detection code to Okular to never call into synctex C code for non-synctex files
- Evaluate feasibility of upstreaming all patches for TeXLive 2018 and using synctex as a library

BUG: 383915
FIXED-IN: 17.12.0

Test Plan:
- No crash in synctex on reloading empty.pdf from ...