public void DrawEllipse(Vector2 center, float radiusHorizontal, float radiusVertical, Color c1, Color c2, float thickness = 1f)
{
    // Account for AA.
    radiusHorizontal += _pixelSize;
    radiusVertical += _pixelSize;

    var topLeft = center + new Vector2(-radiusHorizontal, -radiusVertical);
    var topRight = center + new Vector2(radiusHorizontal, -radiusVertical);
    var bottomRight = center + new Vector2(radiusHorizontal, radiusVertical);
    var bottomLeft = center + new Vector2(-radiusHorizontal, radiusVertical);

    var ps = _pixelSize / (Math.Max(radiusHorizontal, radiusVertical) * 2);

    var u = 1.0f;

    _vertices[_vertexCount + 0] = new VertexShape(new Vector3(topLeft, 0), new Vector2(-u, -u), 0f, c1, c2, thickness, ps);
    _vertices[_vertexCount + 1] = new VertexShape(new Vector3(topRight, 0), new Vector2(u, -u), 0f, c1, c2, thickness, ps);
    _vertices[_vertexCount + 2] = new VertexShape(new Vector3(bottomRight, 0), new Vector2(u, u), 0f, c1, c2, thickness, ps);
    _vertices[_vertexCount + 3] = new VertexShape(new Vector3(bottomLeft, 0), new Vector2(-u, u), 0f, c1, c2, thickness, ps);

    _triangleCount += 2;
    _vertexCount += 4;
    _indexCount += 6;

    if (_triangleCount >= MAX_TRIANGLES)
    {
        Flush();
    }
}