Multi core support

[sfe-SparkFro]

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

micropython-opencv/src/opencv/platforms/rp2350.toolchain.cmake

Line 13 in 2486762

set(OPENCV_DISABLE_THREAD_SUPPORT ON)

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.

[sfe-SparkFro]

Looks like the Pico SDK doesn't currently support std:recursive_mutex, which is needed for multi-core support. Issue created: raspberrypi/pico-sdk#2557