BBCodeText background corner radius becomes square on editing

[charlie-says]

Minor issue, I was just implementing an editable textfield, and noticed that when I applied rounded corners to the background, this became square when you were editing the text.

[rexrainbow]

Text-edit will place an InputText object at the same position of text game object. However, I did not add round-corner feature for InputText. Add this feature to TODO.

[rexrainbow]

Support backgroundCornerRadius parameter, please see this demo, line 23.
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