Multi core support

Early in development, `OPENCV_DISABLE_THREAD_SUPPORT` was set to `ON` (at least for RP2350):

https://github.com/sparkfun/micropython-opencv/blob/2486762b683954312016ed3a28919816b6bd6f17/src/opencv/platforms/rp2350.toolchain.cmake#L13

This was just to get things working, but having multiple threads would be fantastic to support, since it would ~double processing speed on dual-core platforms.