Construct Mat from raw pixel directly

[rainyl]

Discussed in #18

^{Originally posted by gau-nernst April 3, 2024}
Typically we can access raw image data from other library as an Uint8List object. Assuming we know the format of the image (e.g. RGB uint8), what is the best (fast and convenient) way to convert it to OpenCV Mat? Some of the approaches I am thinking of:

• Create an empty Mat with the required size. Iterate over Uint8List and copy data over. For some formats like YUV_420sp, it might be a bit tricky to copy the data correctly.
• Encode in a lossless format (e.g. PNG) and use cv.imdecode() -> seems to be the most convenient, but there is overhead in compression-decompression.
• Allocate C memory with Dart FFI, copy data over, create Mat from C pointer. Need to release the memory after use.

[rainyl]

Finished in https://github.com/rainyl/opencv_dart/releases/tag/v1.0.0