math_ext.cs

using Unity.Mathematics;

public static class math_ext
{
    private static float2 vect2d(float2 p1, float2 p2)
    {
        float2 temp;
        temp.x = (p2.x - p1.x);
        temp.y = -(p2.y - p1.y);
        return temp;
    }

    private static bool vectEquals(float2 a, float2 b)
    {
        return a.x == b.x && a.y == b.y;
    }

    public static bool PointIsInArea(float2 m, float2 A, float2 B, float2 C, float2 D)
    {
        if (vectEquals(A, B) && vectEquals(A, C) && vectEquals(A, D))
        {
            return vectEquals(A, m);
        }

        float2 AB = vect2d(A, B);
        float C1 = -(AB.y * A.x + AB.x * A.y);
        float D1 = (AB.y * m.x + AB.x * m.y) + C1;

        float2 AD = vect2d(A, D);
        float C2 = -(AD.y * A.x + AD.x * A.y);
        float D2 = (AD.y * m.x + AD.x * m.y) + C2;

        float2 BC = vect2d(B, C);
        float C3 = -(BC.y * B.x + BC.x * B.y);
        float D3 = (BC.y * m.x + BC.x * m.y) + C3;

        float2 CD = vect2d(C, D);
        float C4 = -(CD.y * C.x + CD.x * C.y);
        float D4 = (CD.y * m.x + CD.x * m.y) + C4;

        return 0 >= D1 && 0 >= D4 && 0 <= D2 && 0 >= D3;
    }
}

Ported from https://stackoverflow.com/a/42396910/2540278