Cycles: Only calculate global size of split kernel once to avoid changes

Global size depends on memory usage which might change during rendering.
Havent seen it happen but seems possible that this could cause the global
size to be different than what was used for allocating buffers.

intern/cycles/device/device_split_kernel.cpp

@@ -128,26 +128,27 @@ bool DeviceSplitKernel::path_trace(DeviceTask *task,
 		local_size[1] = lsize[1];
 	}
 
-	/* Set gloabl size */
-	size_t global_size[2];
-	{
-		int2 gsize = split_kernel_global_size(kgbuffer, kernel_data, task);
-
-		/* Make sure that set work size is a multiple of local
-		 * work size dimensions.
-		 */
-		global_size[0] = round_up(gsize[0], local_size[0]);
-		global_size[1] = round_up(gsize[1], local_size[1]);
-	}
-
 	/* Number of elements in the global state buffer */
 	int num_global_elements = global_size[0] * global_size[1];
-	assert(num_global_elements % WORK_POOL_SIZE == 0);
 
 	/* Allocate all required global memory once. */
 	if(first_tile) {
 		first_tile = false;
 
+		/* Set gloabl size */
+		{
+			int2 gsize = split_kernel_global_size(kgbuffer, kernel_data, task);
+
+			/* Make sure that set work size is a multiple of local
+			 * work size dimensions.
+			 */
+			global_size[0] = round_up(gsize[0], local_size[0]);
+			global_size[1] = round_up(gsize[1], local_size[1]);
+		}
+
+		num_global_elements = global_size[0] * global_size[1];
+		assert(num_global_elements % WORK_POOL_SIZE == 0);
+
 		/* Calculate max groups */
 
 		/* Denotes the maximum work groups possible w.r.t. current requested tile size. */

intern/cycles/device/device_split_kernel.h

@@ -95,6 +95,9 @@ private:
 	/* Marked True in constructor and marked false at the end of path_trace(). */
 	bool first_tile;
 
+	/* Cached global size */
+	size_t global_size[2];
+
 public:
 	explicit DeviceSplitKernel(Device* device);
 	virtual ~DeviceSplitKernel();