text input selection radius

[ickshonpe]

Objective

Allow rounded corners for text input selection regions.

Fixes #23952

Solution

• Added a selection_radius field to TextCursorStyle, defaults to 0..
• In extract_text_cursor set a corner radius for each selection rect, after clamping for short and inner corners.

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Left rounded inner corners for a follow up, as they are significantly more complicated to implement.

Testing

Added an option to set the selection radius to the multiline_text_input example:

cargo run --example multiline_text_input --features="system_clipboard"

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