Add back dirty bits for pack / unpack buffer binding

BUG=angleproject:2169

Change-Id: I8cee9e1c7565d90022216d62b45f0eec5bc51f14
Reviewed-on: https://chromium-review.googlesource.com/755461
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index 476a139..eef8564 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -390,10 +390,12 @@
 
     // Initialize dirty bit masks
     mTexImageDirtyBits.set(State::DIRTY_BIT_UNPACK_STATE);
+    mTexImageDirtyBits.set(State::DIRTY_BIT_UNPACK_BUFFER_BINDING);
     // No dirty objects.
 
     // Readpixels uses the pack state and read FBO
     mReadPixelsDirtyBits.set(State::DIRTY_BIT_PACK_STATE);
+    mReadPixelsDirtyBits.set(State::DIRTY_BIT_PACK_BUFFER_BINDING);
     mReadPixelsDirtyObjects.set(State::DIRTY_OBJECT_READ_FRAMEBUFFER);
 
     mClearDirtyBits.set(State::DIRTY_BIT_RASTERIZER_DISCARD_ENABLED);