Bug 1727752 - Document DIGroup and fix stale comment. r=mstange

In bug 1548056 we stopped using FLB/ComputeDifferences for invalidation of
filters and transform and switched to ConstructGroupInsideInactive.

Differential Revision: https://phabricator.services.mozilla.com/D123760

gfx/layers/wr/WebRenderCommandBuilder.cpp

@@ -267,6 +267,10 @@ static bool DetectContainerLayerPropertiesBoundsChange(
   return !aGeometry.mBounds.IsEqualEdges(aData->mGeometry->mBounds);
 }
 
+/* A Display Item Group. This represents a set of diplay items that
+ * have been grouped together for rasterization and can be partially
+ * invalidated. It also tracks a number of properties from the environment
+ * that when changed would cause us to repaint like mScale. */
 struct DIGroup {
   // XXX: Storing owning pointers to the BlobItemData in a hash table is not
   // a good choice. There are two better options:
@@ -1293,9 +1297,9 @@ bool Grouper::ConstructItemInsideInactive(
       aGroup->mClippedImageBounds.Intersect(data->mRect);
 
   if (aItem->GetType() == DisplayItemType::TYPE_FILTER) {
-    // If ComputeDifferences finds any change, we invalidate the entire
+    // If ConstructGroupInsideInactive finds any change, we invalidate the
-    // container item. This is needed because blob merging requires the entire
+    // entire container item. This is needed because blob merging requires the
-    // item to be within the invalid region.
+    // entire item to be within the invalid region.
     Matrix m = mTransform;
     mTransform = Matrix();
     bool old = aGroup->mSuppressInvalidations;
@@ -1318,9 +1322,9 @@ bool Grouper::ConstructItemInsideInactive(
     Matrix t2d;
     bool is2D = t.CanDraw2D(&t2d);
     if (!is2D) {
-      // If ComputeDifferences finds any change, we invalidate the entire
+      // If ConstructGroupInsideInactive finds any change, we invalidate the
-      // container item. This is needed because blob merging requires the entire
+      // entire container item. This is needed because blob merging requires the
-      // item to be within the invalid region.
+      // entire item to be within the invalid region.
       bool old = aGroup->mSuppressInvalidations;
       aGroup->mSuppressInvalidations = true;
       mTransform = Matrix();