fix position of background, border and underline shapes in case that the line contains custom objects

[JFormDesigner]

If a line contains a custom object (e.g. image in RichText demo) and you apply background color to text after that image, then the background is painted one character to the left where it should be painted.

The problem is simply that the image is not considered when calculating the start position of the background color.

This PR fixes issue #638

[JFormDesigner]

Steps to reproduce the issue:

1. start RichTextDemo
2. enter some text; e.g. "abc"
3. click "insert image" in toolbar and insert some image (into same line as previous text)
4. enter more text in the same line after the image; e.g. "123"
5. select the "3"
6. click "Text background" in toolbar and choose some color

Now the background color is shown at the "2", but it should at the "3"

[Jugen]

@JFormDesigner thanks for the PR