Fix race condition when mapping a D2D surface while its DrawTarget mutates. (bug 1418348, r=bas)

--HG--
extra : rebase_source : cf16e7f1cea664327b00f01bd42380de672cd9b4

gfx/2d/SourceSurfaceD2D1.cpp

@@ -41,13 +41,18 @@ SourceSurfaceD2D1::IsValid() const
 already_AddRefed<DataSourceSurface>
 SourceSurfaceD2D1::GetDataSurface()
 {
-  HRESULT hr;
+  Maybe<MutexAutoLock> lock;
+  if (mSnapshotLock) {
+    lock.emplace(*mSnapshotLock);
+  }
 
   if (!EnsureRealizedBitmap()) {
     gfxCriticalError() << "Failed to realize a bitmap, device " << hexa(mDevice);
     return nullptr;
   }
 
+  HRESULT hr;
+
   RefPtr<ID2D1Bitmap1> softwareBitmap;
   D2D1_BITMAP_PROPERTIES1 props;
   props.dpiX = 96;