Bug 251136. Fix use of mSurface after it has been deleted. r+sr=blizzard

gfx/src/gtk/nsRenderingContextGTK.cpp

@@ -576,7 +576,7 @@ void nsRenderingContextGTK::UpdateGC()
     if (!gcCache) return;
   }
 
-  mGC = gcCache->GetGC(mSurface->GetDrawable(),
+  mGC = gcCache->GetGC(mOffscreenSurface->GetDrawable(),
                        &values,
                        valuesMask,
                        rgn);