Fix sb/rt size computation

Review URL: http://codereview.appspot.com/4809087/



git-svn-id: http://skia.googlecode.com/svn/trunk@2079 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrRenderTarget.cpp b/gpu/src/GrRenderTarget.cpp
index 7b26811..a5f1216 100644
--- a/gpu/src/GrRenderTarget.cpp
+++ b/gpu/src/GrRenderTarget.cpp
@@ -31,10 +31,11 @@
     } else {
         colorBits = GrBytesPerPixel(fConfig);
     }
-    return (size_t) fAllocatedWidth *
-                    fAllocatedHeight *
-                    colorBits *
-                    GrMax(1,fSampleCnt);
+    uint64_t size = fAllocatedWidth;
+    size *= fAllocatedHeight;
+    size *= colorBits;
+    size *= GrMax(1,fSampleCnt);
+    return (size_t)(size / 8);
 }
 
 void GrRenderTarget::flagAsNeedingResolve(const GrIRect* rect) {
@@ -71,4 +72,4 @@
         fStencilBuffer->wasAttachedToRenderTarget(this);
         fStencilBuffer->ref();
     }
-}
\ No newline at end of file
+}