fix a small bug that caused screenshot to show garbage in some cases

we were not clearing the screen entirely, which caused garbage when
the screen wasn't entirely covered by windows.

Change-Id: Ie9ab9b94eabfa6cafddf45bb14bc733bdc8d35c0
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 7c4790f..825d90b 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -2123,6 +2123,7 @@
 
         // invert everything, b/c glReadPixel() below will invert the FB
         glViewport(0, 0, sw, sh);
+        glScissor(0, 0, sw, sh);
         glMatrixMode(GL_PROJECTION);
         glPushMatrix();
         glLoadIdentity();
@@ -2132,6 +2133,7 @@
         // redraw the screen entirely...
         glClearColor(0,0,0,1);
         glClear(GL_COLOR_BUFFER_BIT);
+
         const Vector< sp<LayerBase> >& layers(mVisibleLayersSortedByZ);
         const size_t count = layers.size();
         for (size_t i=0 ; i<count ; ++i) {
@@ -2169,7 +2171,6 @@
                 result = NO_MEMORY;
             }
         }
-
         glEnable(GL_SCISSOR_TEST);
         glViewport(0, 0, hw_w, hw_h);
         glMatrixMode(GL_PROJECTION);