Prevent gratuitous blueprint saves by not gc'ing when the blueprint h…

…asn't changed (#5793)

### What
Whenever we gc the blueprint, it bumps the generation, which in turn
causes another save. By tracking the last gc, we never re-gc again if
nothing has changed.

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5793)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5793?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5793?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5793)
- [Docs
preview](https://rerun.io/preview/b0543dcc9e0c0d9abd4fe67e940bf101794d0566/docs)
<!--DOCS-PREVIEW-->
- [Examples
preview](https://rerun.io/preview/b0543dcc9e0c0d9abd4fe67e940bf101794d0566/examples)
<!--EXAMPLES-PREVIEW-->
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

crates/re_viewer_context/src/store_hub.rs

@@ -45,8 +45,11 @@ pub struct StoreHub {
     /// Was a recording ever activated? Used by the heuristic controlling the welcome screen.
     was_recording_active: bool,
 
-    // The [`StoreGeneration`] from when the [`EntityDb`] was last saved
+    /// The [`StoreGeneration`] from when the [`EntityDb`] was last saved
     blueprint_last_save: HashMap<StoreId, StoreGeneration>,
+
+    /// The [`StoreGeneration`] from when the [`EntityDb`] was last garbage collected
+    blueprint_last_gc: HashMap<StoreId, StoreGeneration>,
 }
 
 /// Load a blueprint from persisted storage, e.g. disk.
@@ -128,6 +131,7 @@ impl StoreHub {
             was_recording_active: false,
 
             blueprint_last_save: Default::default(),
+            blueprint_last_gc: Default::default(),
         }
     }
 
@@ -547,9 +551,16 @@ impl StoreHub {
                 .chain(self.default_blueprint_by_app_id.values())
             {
                 if let Some(blueprint) = self.store_bundle.get_mut(blueprint_id) {
+                    if self.blueprint_last_gc.get(blueprint_id) == Some(&blueprint.generation()) {
+                        continue; // no change since last gc
+                    }
+
                     // TODO(jleibs): Decide a better tuning for this. Would like to save a
                     // reasonable amount of history, or incremental snapshots.
                     blueprint.gc_everything_but_the_latest_row();
+
+                    self.blueprint_last_gc
+                        .insert(blueprint_id.clone(), blueprint.generation());
                 }
             }
         }
@@ -579,6 +590,8 @@ impl StoreHub {
 
             if app_options.blueprint_gc {
                 blueprint.gc_everything_but_the_latest_row();
+                self.blueprint_last_gc
+                    .insert(blueprint_id.clone(), blueprint.generation());
             }
 
             if let Some(saver) = &self.persistence.saver {