SurfaceFlinger and libgui: Support for child layers.

Add support for parenting Layers in a tree. Layers
follow scene-graph style rules, that is to say:
   1. A child is cropped to the final bounds of the parent.
   2. A child inherits the parent's transform (including position)
   3. A child's Z ordering is relative to the parent and bounded between
      the parents siblings.
   4. A childs lifetime is bounded by it's parents lifetime.

Test: New tests in Transaction_test plus manual testing with later branches.
Change-Id: I96f8ad863665b9a70b6f845561344c297b7e6eff
diff --git a/include/gui/SurfaceComposerClient.h b/include/gui/SurfaceComposerClient.h
index 4356a58..789dc7c 100644
--- a/include/gui/SurfaceComposerClient.h
+++ b/include/gui/SurfaceComposerClient.h
@@ -105,7 +105,8 @@
             uint32_t w,         // width in pixel
             uint32_t h,         // height in pixel
             PixelFormat format, // pixel-format desired
-            uint32_t flags = 0  // usage flags
+            uint32_t flags = 0,  // usage flags
+            SurfaceControl* parent = nullptr // parent
     );
 
     //! Create a virtual display