Rework/simplify ListClipper drop impl

[dbr]

If you call .end on the list clipper, it'll crash when the destructor also calls the same

I don't think this happened in the last imgui-rs release, but happens in latest main branch (e.g 69fb34f)

The long_list example works fine as-is, but crashes if you do this:

$ git diff
diff --git a/imgui-examples/examples/long_list.rs b/imgui-examples/examples/long_list.rs
index 2204c53..fb7bab4 100644
--- a/imgui-examples/examples/long_list.rs
+++ b/imgui-examples/examples/long_list.rs
@@ -26,6 +26,7 @@ fn main() {
                         ui.text(&lots_of_words[row_num as usize]);
                     }
                 }
+                clipper.end();
             });
     });
 }

[dbr]

The crash happens in bool ImGuiListClipper::Step() on this line, because data is NULL:

if (data->StepNo == 0 && table != NULL && !table->IsUnfrozenRows)

The crashing call to step is happening via the Drop impl on ListClipper, which does this:

    fn drop(&mut self) {
        if !self.step() {
            unsafe {
                sys::ImGuiListClipper_destroy(self.list_clipper);
            };
        } else if !thread::panicking() {
            panic!(

This might be a regression in Dear ImGui, but..

Currently the ListClipper drop behaviour seems inconvinient - ignoring this bug, it'll panic if you don't exhaust the token, so I think something like

while clip.step() {
    ui.text(format!("Thing: {}", some_call()?));
}

..would panic in the case some_call()? returns early, which would be surprising at best

Given this..

1. isn't impacting any released imgui-rs version
2. the token is the last one with a panicing drop
3. the upstream API has changed
4. workaround in current main version is simple (just don't call .end(), as per long_list example)

Then it seems like we should just update the ListClipper to be inline with the permissive-dropping and new upstream API

[dbr]

This most likely relates to ocornut/imgui#4822 in some way (although not exactly as that problem was hitting an intentional asser)

[ocornut]

Pushed a fix upstream for this and #611
ocornut/imgui@d9e60d2

[dbr]

Then it seems like we should just update the ListClipper to be inline with the permissive-dropping and new upstream API

This indeed seems like the right thing to do as per #611 (comment)

The drop impl should just call end if it's not already - the extra call to check Step() == false is a bad idea (there's nothing wrong with stopping it half-way through a table, for example)

While changing this, should also investigate if the list clipper should act as an iterator, so instead of

let mut clip = imgui::ListClipper::new(table.total_rows() as i32).begin(ui);

while clip.step() {
    for row_num in clip.display_start()..clip.display_end() {
        ui.text("...")
    }
}

we can instead write:

let mut clip = imgui::ListClipper::new(table.total_rows() as i32).begin(ui);

for row in clip.iter_rows() {
    ui.text("...")
}

Should be easy to retain both interfaces as both are useful