add a way to toggle the h/w composer at runtime

to DISABLE the h/w composer:
  adb shell service call SurfaceFlinger 1008 i32 1

to ENABLE the h/w composer:
  adb shell service call SurfaceFlinger 1008 i32 0

the state is dumped in "dumpsys SurfaceFlinger"

Change-Id: I23e2242d42c6e3fd5261a83332dd900b189e38ce
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 8e286e5..551e8e7 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -382,6 +382,7 @@
                 // don't use a lock for these, we don't care
                 int                         mDebugRegion;
                 int                         mDebugBackground;
+                int                         mDebugDisableHWC;
                 volatile nsecs_t            mDebugInSwapBuffers;
                 nsecs_t                     mLastSwapBufferTime;
                 volatile nsecs_t            mDebugInTransaction;