SurfaceComposerClient BLAST Transactions: Correctly merge mContainsBuffer

If we merge a transaction which does not contain a buffer in to a transaction
which already contained a buffer, the resulting transaction still needs to
have mContainsBuffer=true, otherwise we won't properly cache the buffers.

Bug: 153561718
Test: Existing tests pass. Manual test of BLASTSyncEngine.
Change-Id: Ic63aba390757285730cb19406dc5c6b73d0c3437
diff --git a/libs/gui/SurfaceComposerClient.cpp b/libs/gui/SurfaceComposerClient.cpp
index 2307fbf..8a7412e 100644
--- a/libs/gui/SurfaceComposerClient.cpp
+++ b/libs/gui/SurfaceComposerClient.cpp
@@ -507,7 +507,7 @@
 
     mInputWindowCommands.merge(other.mInputWindowCommands);
 
-    mContainsBuffer = other.mContainsBuffer;
+    mContainsBuffer |= other.mContainsBuffer;
     mEarlyWakeup = mEarlyWakeup || other.mEarlyWakeup;
     other.clear();
     return *this;