clients: keyboard: fix delete before cursor

Aske requested to merge askebm/weston:virtual-keyboard-fixup into main

When editing a text field in chromium deleting the surrounding text with backspace fails with The selection range for surrounding text is invalid

It seems like (start - keyboard->surrounding_text) - keyboard->surrounding_cursor evaluates to -1.

As far as i can tell start - keyboard->surrounding_text evaluates to the index in the surrounding_text char arrays that should be deleted.

Signed-off-by: Aske Bækdal Møller

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