Bug 1576499 - Clarify dest and buffer a little. r=mattwoodrow

The old code was making an autoRestoreTransform around dest and then quickly
overwriting dest with something new. I prefer creating the autoRestoreTransform
for buffer, which doesn't change over the course of this function.

Differential Revision: https://phabricator.services.mozilla.com/D43384

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

gfx/layers/basic/BasicCompositor.cpp

@@ -619,19 +619,19 @@ void BasicCompositor::DrawGeometry(
     gfx::Float aOpacity, const gfx::Matrix4x4& aTransform,
     const gfx::Rect& aVisibleRect, const bool aEnableAA) {
   RefPtr<DrawTarget> buffer = mRenderTarget->mDrawTarget;
-
-  // For 2D drawing, |dest| and |buffer| are the same surface. For 3D drawing,
-  // |dest| is a temporary surface.
-  RefPtr<DrawTarget> dest = buffer;
-
-  AutoRestoreTransform autoRestoreTransform(dest);
+  AutoRestoreTransform autoRestoreTransform(buffer);
 
   Matrix newTransform;
   Rect transformBounds;
   Matrix4x4 new3DTransform;
   IntPoint offset = mRenderTarget->GetOrigin();
 
+  // For 2D drawing, |dest| and |buffer| are the same surface. For 3D drawing,
+  // |dest| is a temporary surface.
+  RefPtr<DrawTarget> dest;
+
   if (aTransform.Is2D()) {
+    dest = buffer;
     newTransform = aTransform.As2D();
   } else {
     // Create a temporary surface for the transform.