Build layer damage bounds for shadows

bug:15538815

Change-Id: I703afeb7e31c28002bd1aff9ce448ec8cdc4e70d
diff --git a/libs/hwui/SpotShadow.cpp b/libs/hwui/SpotShadow.cpp
index 06f6204..82dbe7a 100644
--- a/libs/hwui/SpotShadow.cpp
+++ b/libs/hwui/SpotShadow.cpp
@@ -508,6 +508,7 @@
     computeSpotShadow(isCasterOpaque, light, lightVertexCount, lightCenter, poly,
             polyLength, retStrips);
     retStrips.setMode(VertexBuffer::kTwoPolyRingShadow);
+    retStrips.computeBounds<AlphaVertex>();
 }
 
 /**