Aggressively purge video data when its unused (#7592)

### What

Before, we'd only remove `re_video` objects on memory purges. With this
change we do it immediately when a video is unused for a frame.
Empirically that's the better strategy given that we're already purging
decoders eagerly (which we have to do pretty much because we create new
ones for every little preview) since it takes a lot longer to get decode
a video than it takes to parse the mp4.



https://github.com/user-attachments/assets/2a841caa-6c7b-4651-ac84-d91135ca9989

crates/viewer/re_viewer_context/src/cache/video_cache.rs

@@ -69,6 +69,13 @@ impl VideoCache {
 
 impl Cache for VideoCache {
     fn begin_frame(&mut self, renderer_active_frame_idx: u64) {
+        // Clean up unused video data.
+        self.0.retain(|_row_id, per_key| {
+            per_key.retain(|_, v| v.used_this_frame.load(Ordering::Acquire));
+            !per_key.is_empty()
+        });
+
+        // Of the remaining video data, remove all unused decoders.
         for per_key in self.0.values() {
             for v in per_key.values() {
                 v.used_this_frame.store(false, Ordering::Release);
@@ -80,10 +87,13 @@ impl Cache for VideoCache {
     }
 
     fn purge_memory(&mut self) {
-        self.0.retain(|_row_id, per_key| {
-            per_key.retain(|_, v| v.used_this_frame.load(Ordering::Acquire));
-            !per_key.is_empty()
-        });
+        // We aggressively purge all unused video data every frame.
+        // The expectation here is that parsing video data is fairly fast,
+        // since decoding happens separately.
+        //
+        // As of writing, in a debug wasm build with Chrome loading a 600MiB 1h video
+        // this assumption holds up fine: There is a (sufferable) delay,
+        // but it's almost entirely due to the decoder trying to retrieve a frame.
     }
 
     fn on_store_events(&mut self, events: &[ChunkStoreEvent]) {