Update to OpenCV 4.12.0

[sfe-SparkFro]

https://opencv.org/blog/opencv-4-12-0-is-now-available/

Doesn't seem like huge changes relevant for MicroPython, but always nice to be on the latest version.

[sfe-SparkFro]

Attempting to build gave the following error:

[ 49%] Building CXX object modules/imgproc/CMakeFiles/opencv_imgproc.dir/src/shapedescr.cpp.obj
/home/dryw/micropython-opencv/src/opencv/opencv/modules/imgproc/src/shapedescr.cpp: In function 'void cv::getClosestEllipsePoints(const RotatedRect&, InputArray, OutputArray)':
/home/dryw/micropython-opencv/src/opencv/opencv/modules/imgproc/src/shapedescr.cpp:954:54: error: 'M_PI' was not declared in this scope
  954 |     float theta_rad = static_cast<float>(angle_deg * M_PI / 180);

Not sure what is needed to ensure math constants are available.

[sfe-SparkFro]

I poked through the change log, and looks like it may not be a simple update. For example:

• Find contours speedup opencv/opencv#26834 adds a pre-allocated memory pool, which may need to go into the C heap instead of the GC pool.
• added optional mask to cv::threshold opencv/opencv#26842 adds an optional mask to threshold(), so we'd need to add that as well.

There may be other things like those, I haven't looked in detail yet. But might be best to hold this update until after launch so we have time to verify functionality on everything.