Perform OCR on OCR-only processes

[phu54321]

Spawn multiple _ocrProcess process and do all the OCR job there

Caveats:

• pytorch is not thread-safe, so use of multiprocessing module is necessary
• OCR processes are NOT gracefully terminated. I don't know when to terminate them.

New options:

• --easyocr-workers [number]: # of OCR processes. VRAM usage increases proportionally. IDK if this could be reduced, as EasyOCR doesn't seem to be multiprocessing-safe
• --easyocr-batch-size [number]: Set batch_size parameter of easyocr. Don't expect a lot of performance gain, though.

Before (See GPU usage)

After (See GPU usage)

not a fair comparison: Old implementation wants to use 12GB of ram, so it doesn't fit in my 3060. New implementation allows you to configure the amount of ram you want.

I'm seeing a speedup of 4x, but this may vary for people with lots of VRAMs. Further testing needed.

Fixes #3

[phu54321]

Force-pushed typo fix :)

[jbarlow83]

Thanks - I added child process cleanup.