Renderer : Avoid deprecated shared_ptr::unique()

This was deprecated in c++17 and removed in c++20 because it gives
potentially incorrect results in a multithreaded environment. All of
our uses are noted as not being able to be called by multiple threads,
so the comparison `use_count() == 1` is valid.

src/GafferCycles/IECoreCyclesPreview/Renderer.cpp

@@ -1571,7 +1571,7 @@ class InstanceCache : public IECore::RefCounted
 
 			for( Geometry::iterator it = m_geometry.begin(), eIt = m_geometry.end(); it != eIt; ++it )
 			{
-				if( it->second.unique() )
+				if( it->second.use_count() == 1 )
 				{
 					// Only one reference - this is ours, so
 					// nothing outside of the cache is using the
@@ -1846,7 +1846,7 @@ class CameraCache : public IECore::RefCounted
 			vector<IECore::MurmurHash> toErase;
 			for( Cache::iterator it = m_cache.begin(), eIt = m_cache.end(); it != eIt; ++it )
 			{
-				if( it->second.unique() )
+				if( it->second.use_count() == 1 )
 				{
 					// Only one reference - this is ours, so
 					// nothing outside of the cache is using the

src/IECoreArnold/Renderer.cpp

@@ -2276,7 +2276,7 @@ class InstanceCache : public IECore::RefCounted
 			vector<IECore::MurmurHash> toErase;
 			for( Cache::iterator it = m_cache.begin(), eIt = m_cache.end(); it != eIt; ++it )
 			{
-				if( it->second.unique() )
+				if( it->second.use_count() == 1 )
 				{
 					// Only one reference - this is ours, so
 					// nothing outside of the cache is using the

src/IECoreDelight/Renderer.cpp

@@ -970,7 +970,7 @@ class InstanceCache : public IECore::RefCounted
 			vector<IECore::MurmurHash> toErase;
 			for( Cache::iterator it = m_cache.begin(), eIt = m_cache.end(); it != eIt; ++it )
 			{
-				if( it->second.unique() )
+				if( it->second.use_count() == 1 )
 				{
 					// Only one reference - this is ours, so
 					// nothing outside of the cache is using the