Document the implementation of saveLayer().

The implementation is simple but tricky. Leave explanations to my
future self so that I don't invent a time machine to come back to
now and slap my self.

This change also simplifies the way the GL blending function is
chosen when compositing a layer. It reuses existing OpenGLRenderer
APIs and is easier to understand.

Change-Id: I1b9cf8c5d51e09836d85b8cf157a1c284aa65c59
diff --git a/libs/hwui/OpenGLRenderer.h b/libs/hwui/OpenGLRenderer.h
index 12ec276..3126754 100644
--- a/libs/hwui/OpenGLRenderer.h
+++ b/libs/hwui/OpenGLRenderer.h
@@ -176,8 +176,7 @@
      * @paran ignoreBlending True if the blending is set by the caller
      */
     void drawColorRect(float left, float top, float right, float bottom,
-    		int color, SkXfermode::Mode mode, bool ignoreTransform = false,
-    		bool ignoreBlending = false);
+            int color, SkXfermode::Mode mode, bool ignoreTransform = false);
 
     /**
      * Draws a textured rectangle with the specified texture. The specified coordinates