feat(example): improve appearance of dark mode filter

[TechnicJelle]

Hi there!
I love the library, it's very good.

However, I felt like the default dark mode filter could be improved somewhat.
It just only inverted the colours, which made the sea orange and the trees purple.

So I've figured out a nicer-looking dark mode filter!
It inverts the colours first, but then hue-rotates by 180 degrees to make the colours look normal again!

Thanks to my friend @Stvff for helping me with these calculations!

You can see it in action here:

Light:

Old dark mode:

New dark mode:

This hard-coded matrix was generated by the following Dart code:

// Using the `ml_linalg` package for the Matrix maths
//  https://pub.dev/packages/ml_linalg

// Matrices from https://www.w3.org/TR/filter-effects-1/#feColorMatrixElement
Matrix mat1 = Matrix.fromList([
  [0.213, 0.715, 0.072],
  [0.213, 0.715, 0.072],
  [0.213, 0.715, 0.072],
]);
Matrix mat2 = Matrix.fromList([
  [0.787, -0.715, -0.072],
  [-0.213, 0.285, -0.072],
  [-0.213, -0.715, 0.928],
]);
Matrix mat3 = mat1 - mat2;
final hueRotator3x3 = mat3.asFlattenedList;

Matrix hueRotator5x5 = Matrix.fromList([
  [hueRotator3x3[0], hueRotator3x3[1], hueRotator3x3[2], 0, 0],
  [hueRotator3x3[3], hueRotator3x3[4], hueRotator3x3[5], 0, 0],
  [hueRotator3x3[6], hueRotator3x3[7], hueRotator3x3[8], 0, 0],
  [0, 0, 0, 1, 0],
  [0, 0, 0, 0, 1],
]);

Matrix inverter5x5 = Matrix.fromList([
  [-1, 0, 0, 0, 255],
  [0, -1, 0, 0, 255],
  [0, 0, -1, 0, 255],
  [0, 0, 0, 1, 0],
  [0, 0, 0, 0, 1],
]);

Matrix result = inverter5x5 * hueRotator5x5;
final values = result.asFlattenedList;

final ColorFilter nicerInvertFilter = ColorFilter.matrix([
  values[0], values[1], values[2], values[3], values[4], // R
  values[5], values[6], values[7], values[8], values[9], // G
  values[10], values[11], values[12], values[13], values[14], // B
  values[15], values[16], values[17], values[18], values[19], // A
]);
print(nicerInvertFilter);

[josxha]

Thanks for this awesome pull request!

[JaffaKetchup]

LGTM, thanks!

[TechnicJelle]

Thanks for accepting it! :D