Use LUT for computing final shadow alpha

bug:27415250

Significantly reduces shadow fragment shader computation.

Change-Id: Ie9b3c712700754b3734d0ae9cda8751c298fc59e
diff --git a/libs/hwui/Caches.cpp b/libs/hwui/Caches.cpp
index eaa1c33..949ad45 100644
--- a/libs/hwui/Caches.cpp
+++ b/libs/hwui/Caches.cpp
@@ -74,6 +74,7 @@
 
     mPixelBufferState = new PixelBufferState();
     mTextureState = new TextureState();
+    mTextureState->constructTexture(*this);
 
     return true;
 }