Added unit tests for scan window calculation

+import 'package:flutter/painting.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mobile_scanner/src/scan_window_calculation.dart';
+
+void main() {
+  group('Scan window relative to texture', () {
+    group('Widget (landscape) smaller than texture (portrait)', () {
+      const textureSize = Size(480.0, 640.0);
+      const widgetSize = Size(432.0, 256.0);
+      final ctx = ScanWindowTestContext(
+        textureSize: textureSize,
+        widgetSize: widgetSize,
+        scanWindow: Rect.fromLTWH(
+          widgetSize.width / 4,
+          widgetSize.height / 4,
+          widgetSize.width / 2,
+          widgetSize.height / 2,
+        ),
+      );
+
+      test('wl tp: BoxFit.none', () {
+        ctx.testScanWindow(BoxFit.none, const Rect.fromLTRB(0.275, 0.4, 0.725, 0.6));
+      });
+
+      test('wl tp: BoxFit.fill', () {
+        ctx.testScanWindow(BoxFit.fill, const Rect.fromLTRB(0.25, 0.25, 0.75, 0.75));
+      });
+
+      test('wl tp: BoxFit.fitHeight', () {
+        ctx.testScanWindow(BoxFit.fitHeight, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+
+      test('wl tp: BoxFit.fitWidth', () {
+        ctx.testScanWindow(BoxFit.fitWidth, const Rect.fromLTRB(0.25, 0.38888888888888895, 0.75, 0.6111111111111112));
+      });
+
+      test('wl tp: BoxFit.cover', () {
+        // equal to fitWidth
+        ctx.testScanWindow(BoxFit.cover, const Rect.fromLTRB(0.25, 0.38888888888888895, 0.75, 0.6111111111111112));
+      });
+
+      test('wl tp: BoxFit.contain', () {
+        // equal to fitHeigth
+        ctx.testScanWindow(BoxFit.contain, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+
+      test('wl tp: BoxFit.scaleDown', () {
+        // equal to fitHeigth, contain
+        ctx.testScanWindow(BoxFit.scaleDown, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+    });
+
+    group('Widget (landscape) smaller than texture and texture (landscape)', () {
+      const textureSize = Size(640.0, 480.0);
+      const widgetSize = Size(320.0, 120.0);
+      final ctx = ScanWindowTestContext(
+        textureSize: textureSize,
+        widgetSize: widgetSize,
+        scanWindow: Rect.fromLTWH(
+          widgetSize.width / 4,
+          widgetSize.height / 4,
+          widgetSize.width / 2,
+          widgetSize.height / 2,
+        ),
+      );
+
+      test('wl tl: BoxFit.none', () {
+        ctx.testScanWindow(BoxFit.none, const Rect.fromLTRB(0.375, 0.4375, 0.625, 0.5625));
+      });
+
+      test('wl tl: BoxFit.fill', () {
+        ctx.testScanWindow(BoxFit.fill, const Rect.fromLTRB(0.25, 0.25, 0.75, 0.75));
+      });
+
+      test('wl tl: BoxFit.fitHeight', () {
+        ctx.testScanWindow(BoxFit.fitHeight, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+
+      test('wl tl: BoxFit.fitWidth', () {
+        ctx.testScanWindow(BoxFit.fitWidth, const Rect.fromLTRB(0.25, 0.375, 0.75, 0.625));
+      });
+
+      test('wl tl: BoxFit.cover', () {
+        // equal to fitWidth
+        ctx.testScanWindow(BoxFit.cover, const Rect.fromLTRB(0.25, 0.375, 0.75, 0.625));
+      });
+
+      test('wl tl: BoxFit.contain', () {
+        // equal to fitHeigth
+        ctx.testScanWindow(BoxFit.contain, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+
+      test('wl tl: BoxFit.scaleDown', () {
+        // equal to fitHeigth, contain
+        ctx.testScanWindow(BoxFit.scaleDown, const Rect.fromLTRB(0.0, 0.25, 1.0, 0.75));
+      });
+    });
+  });
+}
+
+class ScanWindowTestContext {
+  final Size textureSize;
+  final Size widgetSize;
+  final Rect scanWindow;
+
+  ScanWindowTestContext({
+    required this.textureSize,
+    required this.widgetSize,
+    required this.scanWindow,
+  });
+
+  void testScanWindow(BoxFit fit, Rect expected) {
+    final actual = calculateScanWindowRelativeToTextureInPercentage(
+      fit,
+      scanWindow,
+      textureSize,
+      widgetSize,
+    );
+
+    // don't use expect(actual, expected) because Rect.toString() only shows one digit after the comma which can be confusing
+    expect(actual.left, expected.left);
+    expect(actual.top, expected.top);
+    expect(actual.right, expected.right);
+    expect(actual.bottom, expected.bottom);
+  }
+}