use Mat::Ptr in Mat.ptrAt

[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

Much more acceptable now.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.17%. Comparing base (c98e506) to head (4d2f562). Report is 6 commits behind head on main.

Files	Patch %	Lines
lib/src/core/mat.dart	75.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #193 +/- ## ========================================== - Coverage 91.18% 91.17% -0.02% ========================================== Files 57 57 Lines 8394 8393 -1 ========================================== - Hits 7654 7652 -2 - Misses 740 741 +1 ```

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