stbt.ocr: Move engine & mode parameter checks to `_tesseract`
stbt.ocr: Move engine & mode parameter checks to `_tesseract`

...from `_tesseract_subprocess`. This allows me to log the values to the
ImageLogger, which fixes the debug output when `_tesseract` returns
early without calling `_tesseract_subprocess` (this only happens if the
specified region doesn't overlap the source frame).

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester