FrontalFaceDetector cause MemoryCorruptionException in multithread usage

[matteofabbri]

• implement a code that find face
• invoke the code with a big bunch of different threads
• Fatal MemoryCorruptionExecption, process die and there is no way to recover

[takuya-takeuchi]

@matteofabbri
You can not share FRDN instance between threads.
So you must

• Create some FaceRecognition instance
• Use multiprocess

[matteofabbri]

@takuya-takeuchi if you take a look at my PR i fixed the issue.
Making different issues dont fix the problem, due to DlibDotNet internal issues.
ASAP i will fix other not thredsafe components

[takuya-takeuchi]

You may be able to avoid excpetion for FaceLocation but other function may face to same issue.
Should we generate internal instance when function is invoked every time?
I guess it is not good.

[matteofabbri]

In the path of detecting faces, extracting encodings and recognize a subject in those encoding there are no other thread unsafe regions.
Unless you select a different kind of faceselector that i will try to fix asap.
On the test i runned, the whole path takes more that 24seconds for serving a thread using the lock to access the FRDN and 400ms with the fix that i applied