Add a LayerScreenshot

A LayerScreenshot is a special type of layer that contains a screenshot of
the screen acquired when its created. It works just like LayerDim.

Bug: 5446982
Change-Id: I7814aff2380e7e146937f2b641907be2a30c76cc
diff --git a/include/surfaceflinger/ISurfaceComposer.h b/include/surfaceflinger/ISurfaceComposer.h
index ea022a6..e7a33f1 100644
--- a/include/surfaceflinger/ISurfaceComposer.h
+++ b/include/surfaceflinger/ISurfaceComposer.h
@@ -53,6 +53,7 @@
         eFXSurfaceNormal    = 0x00000000,
         eFXSurfaceBlur      = 0x00010000,
         eFXSurfaceDim       = 0x00020000,
+        eFXSurfaceScreenshot= 0x00030000,
         eFXSurfaceMask      = 0x000F0000,
     };