[WinForms] Incorrect Tree Nodes drawing at the `OwnerDrawText` mode (#18605)

Assume we have the following form `MyForm`:
```
using System;
using System.Drawing;
using System.Windows.Forms;

namespace test_TreeView
{
    public class MyForm : Form
    {
        TreeView treeView = new TreeView()

        public MyForm()
        {
            treeView.Nodes.Add("root node");
            treeView.Nodes[0].Nodes.Add("first child node");

            treeView.BackColor = Color.Gray;
            treeView.LineColor = Color.Yellow;

            treeView.DrawMode = TreeViewDrawMode.OwnerDrawText;
            treeView.DrawNode += treeView_DrawNode;

            Controls.Add(treeView);
        }

        private void treeView_DrawNode(object sender, DrawTreeNodeEventArgs e)
        {
            var brush = new SolidBrush(Color.FromArgb(100, 255, 0, 0));
            e.Graphics.FillRectangle(brush, e.Bounds);
        }
    }
}
```
The displayed result is
![image](https://user-images.githubusercontent.com/12382656/73263417-92872980-41e1-11ea-8f7c-7cecbb376b60.png)
One can see couple drawing issues:
* White rectangles under each `TreeNode`.
* The area of custom drawing (red rectangles) is shifted with respect to this white rectangles.

Current PR proposes the fix of this issues.
Co-authored-by: monojenkins <jo.shields+jenkins@xamarin.com>

1 files changed, 0 insertions, 0 deletions

diff --git a/external/api-snapshot b/external/api-snapshot
-Subproject 5f6d34ae3f35a3063f6084845fcdbd80393b3e1
+Subproject 5b8247e289d17cef9ff645e21d896ea75bfe632