The main logic is placed in blendthumb_extract.cc and is built as static
library. For windows there is DLL which is registered during blender
install and which then reads and generates thumbnails.
For other platforms there is blender-thumbnailer executable file which
takes blend file as an input and generates PNG file. As a result
Python script blender-thumbnailer.py is no longer needed.
The thumbnail extractor shares the same code-path as Blenders file
reading, so there is no need to duplicate any file reading logic.
This means reading compressed blend files is supported (broken since
the recent move Zstd compression - D5799).
This resolves T63736.