Currently, when working with callbacks on an InputText, it's possible for TextCallbackData::selection() to return a range with start > end, eg. when selecting text by dragging from right to left. One resulting problem is that when you do this, TextCallbackData::has_selection() will return false, because the implementation is simply

pub fn has_selection(&self) -> bool {
    !self.selection().is_empty()
}

with stdlib's implementation of Range::is_empty() being

pub fn is_empty(&self) -> bool {
    !(self.start < self.end)
}

A bigger problem is that calling TextCallbackData::selected() with the selection in this state will cause a guaranteed panic, because the selection range is used to index the input buffer.

This MR provides one possible solution by ensuring the Range returned by TextCallbackData::selection() always satisfies start <= end.