Bug 1556470. Add a Flush() to DrawTargetD2D1::IntoLuminanceSource(). r=bas

I needed this change to make things work with my CreateClippedDrawTarget. It
seems reasonable but I can't justify it rigorously.

Differential Revision: https://phabricator.services.mozilla.com/D33527

--HG--
extra : moz-landing-system : lando

gfx/2d/DrawTargetD2D1.cpp

@@ -155,6 +155,8 @@ already_AddRefed<SourceSurface> DrawTargetD2D1::IntoLuminanceSource(
     return DrawTarget::IntoLuminanceSource(aLuminanceType, aOpacity);
   }
 
+  Flush();
+
   mLuminanceEffect->SetInput(0, mBitmap);
 
   RefPtr<ID2D1Image> luminanceOutput;