Cycles: Fix possible race condition when generating Beckmann table

Two issues here:

- Checking table size to be non-zero is not a proper way to go here. This is
  because we first resize the table and then fill it in. So it was possible that
  non-initialized table was used.

  Trickery with using temporary memory and then doing table.swap() might work,
  but we can not guarantee that table size will be set after the data pointer.

- Mutex guard was useless, because every thread was using own mutex. Need to
  make mutex guard static so all threads are using same mutex.

intern/cycles/render/shader.cpp

@@ -33,7 +33,9 @@
 
 CCL_NAMESPACE_BEGIN
 
+thread_mutex ShaderManager::lookup_table_mutex;
 vector<float> ShaderManager::beckmann_table;
+bool ShaderManager::beckmann_table_ready = false;
 
 /* Beckmann sampling precomputed table, see bsdf_microfacet.h */
 
@@ -489,10 +491,11 @@ void ShaderManager::device_update_common(Device *device,
 
 	/* beckmann lookup table */
 	if(beckmann_table_offset == TABLE_OFFSET_INVALID) {
-		if(beckmann_table.size() == 0) {
+		if(!beckmann_table_ready) {
 			thread_scoped_lock lock(lookup_table_mutex);
-			if(beckmann_table.size() == 0) {
+			if(!beckmann_table_ready) {
 				beckmann_table_build(beckmann_table);
+				beckmann_table_ready = true;
 			}
 		}
 		beckmann_table_offset = scene->lookup_tables->add_table(dscene, beckmann_table);

intern/cycles/render/shader.h

@@ -204,8 +204,9 @@ protected:
 	typedef unordered_map<ustring, uint, ustringHash> AttributeIDMap;
 	AttributeIDMap unique_attribute_id;
 
-	thread_mutex lookup_table_mutex;
+	static thread_mutex lookup_table_mutex;
 	static vector<float> beckmann_table;
+	static bool beckmann_table_ready;
 
 	size_t beckmann_table_offset;