Partial unclipped save layer support

Not yet implemented in renderer.

Change-Id: I491ec6e7886bfa313d1db71dd5981690d45b78a9
diff --git a/libs/hwui/BakedOpRenderer.cpp b/libs/hwui/BakedOpRenderer.cpp
index a0d5fae..4aebe3c 100644
--- a/libs/hwui/BakedOpRenderer.cpp
+++ b/libs/hwui/BakedOpRenderer.cpp
@@ -236,12 +236,13 @@
 }
 
 void BakedOpRenderer::prepareRender(const Rect* dirtyBounds, const ClipBase* clip) {
-    // prepare scissor / stencil
+    // Prepare scissor (done before stencil, to simplify filling stencil)
     mRenderState.scissor().setEnabled(clip != nullptr);
     if (clip) {
         mRenderState.scissor().set(mRenderTarget.viewportHeight, clip->rect);
     }
 
+    // If stencil may be used for clipping, enable it, fill it, or disable it as appropriate
     if (CC_LIKELY(!Properties::debugOverdraw)) {
         // only modify stencil mode and content when it's not used for overdraw visualization
         if (CC_UNLIKELY(clip && clip->mode != ClipMode::Rectangle)) {