CMake: execute `code_snapshot` separately for each file we want to
scan so we can create timestamps and generate PNG images only if
the source file (e.g. unicode.dox) was modified.
documentation/src/unicode.dox: remove output folder `generated`, use
only the filename. CMake sets the current working directory as needed.
util/code_snapshot.cxx: format and improve comments, reset code buffer
so we can generate multiple images per input file.
The Doxygen-to-pdf toolchain can not easily generate
pdf's with Japanese and Chinese characters. This patch
generates code blocks by rendering them in FLTK.
