Bug 799768: Only bother computing an invalid rect if it affects composition and enable omtc for X11 b2g builds. r=mattwoodrow

b2g/app/nsBrowserApp.cpp

@@ -158,6 +158,10 @@ int main(int argc, char* argv[])
 {
   char exePath[MAXPATHLEN];
 
+#if defined(MOZ_X11)
+  putenv("MOZ_USE_OMTC=1");
+#endif
+
   nsresult rv = mozilla::BinaryPath::Get(argv[0], exePath);
   if (NS_FAILED(rv)) {
     Output("Couldn't calculate the application directory.\n");

layout/base/nsDisplayList.cpp

@@ -1061,7 +1061,7 @@ void nsDisplayList::PaintForFrame(nsDisplayListBuilder* aBuilder,
   NotifySubDocInvalidationFunc computeInvalidFunc =
     presContext->MayHavePaintEventListenerInSubDocument() ? nsPresContext::NotifySubDocInvalidation : 0;
   bool computeInvalidRect = (computeInvalidFunc ||
-                             (layerManager->GetBackendType() == LAYERS_BASIC)) &&
+                             !layerManager->IsCompositingCheap()) &&
                             widgetTransaction;
 
   nsAutoPtr<LayerProperties> props(computeInvalidRect ?