Bug 1541078 - Add pre/post frames hooks into save_capture r=kvark

There's two ways we could get around this. We could add a path
around the prepared_for_frames assertion in GpuCache::end_frame,
or we can do this, and leave the TODO explicit with the
assertion. I took the latter approach because we can clear
the GpuCache / TextureCache through other routes than frame
building anyway, so we could hit this failure path before
any of the patches in this bug landed.

Differential Revision: https://phabricator.services.mozilla.com/D29884

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/gpu_cache.rs

@@ -728,6 +728,7 @@ impl GpuCache {
         let mut cache = Self::new();
         let mut now = FrameStamp::first(DocumentId::new(IdNamespace(1), 1));
         now.advance();
+        cache.prepared_for_frames = true;
         cache.begin_frame(now);
         cache
     }

gfx/wr/webrender/src/render_backend.rs

@@ -1665,6 +1665,10 @@ impl RenderBackend {
         }
         let config = CaptureConfig::new(root, bits);
 
+        if config.bits.contains(CaptureBits::FRAME) {
+            self.prepare_for_frames();
+        }
+
         for (&id, doc) in &mut self.documents {
             debug!("\tdocument {:?}", id);
             if config.bits.contains(CaptureBits::SCENE) {
@@ -1706,6 +1710,14 @@ impl RenderBackend {
             config.serialize(&doc.data_stores, data_stores_name);
         }
 
+        if config.bits.contains(CaptureBits::FRAME) {
+            // TODO: there is no guarantee that we won't hit this case, but we want to
+            // report it here if we do. If we don't, it will simply crash in
+            // Renderer::render_impl and give us less information about the source.
+            assert!(!self.requires_frame_build(), "Caches were cleared during a capture.");
+            self.bookkeep_after_frames();
+        }
+
         debug!("\tscene builder");
         self.scene_tx.send(SceneBuilderRequest::SaveScene(config.clone())).unwrap();