Bug 1380649 - Part 2. Ensure SourceSurfaceVolatileData does not forget its purged state. r=jrmuizel

Currently if SourceSurfaceVolatileData::Map fails due to being purged,
we expect that the surface will be discarded by the caller. This has not
consistently been the case, and as such, we should ensure we do not
forget if a buffer was previously purged when we reacquire it. Since we
do not at this time support repopulating an already allocated buffer
with new data, we cannot reset this state once it has been set.

gfx/layers/SourceSurfaceVolatileData.h

@@ -32,6 +32,7 @@ public:
     , mStride(0)
     , mMapCount(0)
     , mFormat(SurfaceFormat::UNKNOWN)
+    , mWasPurged(false)
   {
   }
 
@@ -66,10 +67,14 @@ public:
   bool Map(MapType, MappedSurface *aMappedSurface) override
   {
     MutexAutoLock lock(mMutex);
+    if (mWasPurged) {
+      return false;
+    }
     if (mMapCount == 0) {
       mVBufPtr = mVBuf;
     }
     if (mVBufPtr.WasBufferPurged()) {
+      mWasPurged = true;
       return false;
     }
     aMappedSurface->mData = mVBufPtr;
@@ -82,6 +87,7 @@ public:
   {
     MutexAutoLock lock(mMutex);
     MOZ_ASSERT(mMapCount > 0);
+    MOZ_ASSERT(!mWasPurged);
     if (--mMapCount == 0) {
       mVBufPtr = nullptr;
     }
@@ -100,6 +106,7 @@ private:
   RefPtr<VolatileBuffer> mVBuf;
   VolatileBufferPtr<uint8_t> mVBufPtr;
   SurfaceFormat mFormat;
+  bool mWasPurged;
 };
 
 } // namespace gfx