Add CompositorWorkspace::setExecutionMask

In OgreNext <= 2.2, mExecutionMask had to be immutable after creating
the Workspace instance.

However after >= 2.3 there was no such hard requirement anymore, as GPU
barriers are analyzed dynamically.
Thus expose the setter.

OgreMain/include/Compositor/OgreCompositorWorkspace.h

@@ -334,6 +334,8 @@ namespace Ogre
         const Vector4 &getViewportModifier() const { return mViewportModifier; }
         void           setViewportModifier( const Vector4 &modifier ) { mViewportModifier = modifier; }
 
+        uint8 setExecutionMask( uint8 executionMask ) { mExecutionMask = executionMask; }
+
         uint8 getExecutionMask() const { return mExecutionMask; }
 
         void _notifyBarriersDirty();