RichTextBox does not repaint border on resize and does not support custom BackColor

[marius-klimantavicius]

Platform windows.

Steps to reproduce:

1. Enable visual styles
2. Add a RichTextBox with Dock = Fill, set BackColor to some value
3. Run app
4. Resize the form - grow

Expected behavior:

1. After resizing the control is rendered correctly (border is fully rendered and back color is the one that was set via BackColor)

Actual result:

1. After form has grown part of the RichTextBox border is missing
2. BackColor is default color

Fixes:

1. I added XplatUI.SendMessage(Handle, Msg.WM_NCPAINT, IntPtr.Zero, IntPtr.Zero); to TextBoxBase.SetBoundsCore (maybe SetStyle(ControlStyles.ResizeRedraw, true) would suffice but I haven't tested that)
2. Updated ThemeVisualStyles.TextBoxBaseFillBackground to use base.TextBoxBaseFillBackground (textBoxBase, g, clippingArea) if back color is set:

if (!RenderClientAreas ||
		    !TextBoxBaseShouldPaint (textBoxBase) ||
		    textBoxBase.backcolor_set // <-- THIS condition
		    ) {
			base.TextBoxBaseFillBackground (textBoxBase, g, clippingArea);
			return;
		}