Re-do update to 1.81 with old submodule setup

Update docs to match

docs/upgrading-imgui.md

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+# Updating to new imgui versions
+
+This document covers how to upgrade imgui-rs to a new version of the upstream C++ library.
+
+The process is much the same to build imgui-rs for a tagged release (as shown) as it is for any arbitrary revision (such as one on a different branch)
+
+
+## Step by step
+
+1. Ensure the submodules are populated (`git submodule init` and `git submodule update --recursive`)
+
+2. Check out the desired version of the `imgui-sys/third-party/imgui/` submodule
+
+    $ pwd
+    .../imgui-sys/third-party/imgui
+    $ git checkout v1.81
+    Previous HEAD position was 58075c44 Version 1.80
+    HEAD is now at 4df57136 Version 1.81
+
+3. Ensure `luajit` is installed, as this is required by cimgui's generator.
+
+    $ luajit --help
+
+4. Check out the `cimgui` project somewhere, as we use use the generator within this
+
+    $ git clone --recursive https://github.com/cimgui/cimgui.git /tmp/cimgui
+
+5. Ensure the `imgui` submodule within `cimgui` is pointing to the same revision as in `imgui-rs`
+
+    $ cd /tmp/cimgui/imgui
+    $ git checkout v1.81
+    HEAD is now at 4df57136 Version 1.81
+
+6. Back in `imgui-rs/imgui-sys/third-party/` - run the `update-cimgui-output.sh` helper script to execute cimgui's generator
+
+    $ pwd
+    .../imgui-sys/third-party
+    $ ./update-cimgui-output.sh /tmp/cimgui/
+    [...]
+    copyfile	./output/cimgui.h	../cimgui.h
+    copyfile	./output/cimgui.cpp	../cimgui.cpp
+    all done!!
+
+  This updates various files in the imgui-sys folder like `cimgui.cpp`, `definitions.json` and so on
+
+  With this step, we now have new C bindings to the desired verison of Dear ImGui.
+
+7. Back in the root of the imgui-rs repo, run `cargo xtask bindgen`
+
+    $ cargo xtask bindgen
+        Finished dev [unoptimized + debuginfo] target(s) in 0.04s
+         Running `target/debug/xtask bindgen`
+    Executing bindgen [output = .../imgui-rs/imgui-sys/src/bindings.rs]
+    Success [output = .../imgui-rs/imgui-sys/src/bindings.rs]
+    Executing bindgen [output = .../imgui-rs/imgui-sys/src/wasm_bindings.rs]
+    Success [output = .../imgui-rs/imgui-sys/src/wasm_bindings.rs]
+
+  This requires bindgen to be installed (`cargo install bindgen` should do it)
+
+  This step generates `imgui-sys/src/bindings.rs` which is used by `imgui/src/*`
+
+8. Run `cargo build` and fix any errors from upstream.
+
+9. Run the tests with `cargo test`.
+
+10. Try running one of the examples
+
+    cargo run --example test_window_impl
+
+
+## Common sources of problems
+
+### Function changes
+
+Check the upstream imgui release notes for the new versions, as they detail any breaking changes.
+
+If functions have been renamed, the required changes to the bindings are usually simple.
+
+If functions have been removed, the changes are usually also simple but the implications may require some thought. Note by default `cimgui` generator will exclude any obsolete API.
+
+If new function overloads are added - for example `imgui::Thing()` existed but `imgui::Thing(float)` was added - `bindings.rs` will previously have contained only `igThing`, but will now contain `igThingNil()` and `igThingFloat(...)`
+
+### Memory layout changes
+
+It is common for upstream to add/remove/reorder fields, so the bindings will compile but the memory layout will not match - which will (hopefully) result in the bindings causing a segfault. These are not tagged as breaking changes in the release notes.
+
+The `*_memory_layout` tests when running `cargo test` should catch these (if they are created for every relevant struct!)
+
+The fix for this is usually to compare the struct in (read-only) `imgui-sys/src/bindings.rs` compared to the relevant struct in `imgui/src/...` - the ordering and data-types must match, but the names do not (structs in `imgui/src/...` should use conventional Rust naming/casing)