graphics/opencv: add TESSERACT option

Tesseract support was explicitly turned off previously,
because in a dirty build environment it would pick up
an existing installation and then break. In the meantime,
opencv and leptonica have been updated, and Tesseract
behaves well in combination with opencv.

Make it an option, default off (corresponding to previous
behavior). While here, hand off to desktop@ at tcberner's request.

PR: 259930