export Mat properies as leaf functions

[rainyl]

fix: #190

test Code

```dart import 'package:opencv_dart/opencv_dart.dart' as cv; void main(List args) { final mat = cv.Mat.zeros(3840, 2160, cv.MatType.CV_32SC3); final sw = Stopwatch(); { sw.reset(); sw.start(); for (var row = 0; row < mat.rows; row++) { for (var col = 0; col < mat.cols; col++) { mat.at(row, col); } } sw.stop(); print('Mat(${mat.rows}, ${mat.cols}, ${mat.channels}).at: ${sw.elapsedMilliseconds}ms'); } { var currentPix = 0; mat.set(mat.rows - 1, mat.cols - 1, cv.Vec3i(241, 241, 241)); sw.reset(); sw.start(); mat.forEachPixel((_, __, pix) => currentPix = pix[0].toInt()); sw.stop(); print('Mat(${mat.rows}, ${mat.cols}, ${mat.channels}).iterPixel At: ${sw.elapsedMilliseconds}ms'); } { sw.reset(); sw.start(); for (var row = 0; row < mat.rows; row++) { for (var col = 0; col < mat.cols; col++) { mat.set(row, col, 1); } } sw.stop(); print('Mat(${mat.rows}, ${mat.cols}, ${mat.channels}).set: ${sw.elapsedMilliseconds}ms'); } { sw.reset(); sw.start(); mat.forEachPixel((_, __, pix) => pix[0] = 241); sw.stop(); print('Mat(${mat.rows}, ${mat.cols}, ${mat.channels}).iterPixel set: ${sw.elapsedMilliseconds}ms'); } } ```

item	isLeaf=false	isLeaf=true	isLeaf=true + uchar *Mat_Ptr_u8()	isLeaf=true + native-assets	isLeaf=true + native-assets + uchar *Mat_Ptr_u8()
at	691ms	561ms	325ms	472ms	301ms
forEachPixel At	177ms	176ms	179ms	184ms	183ms
set	556ms	430ms	267ms	437ms	245ms
forEachPixel Set	175ms	162ms	177ms	179ms	175ms

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.18%. Comparing base (22318bb) to head (c98e506). Report is 5 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #191 +/- ## ======================================= Coverage 91.18% 91.18% ======================================= Files 57 57 Lines 8394 8394 ======================================= Hits 7654 7654 Misses 740 740 ```

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