Cycles: Fix race condition happening in progress utility

This is not enough to mutex-guard modification code of integer values,
since this operation is NOT atomic. This is not even safe for a single
byte data types.

For now guarded the getter functions, similar to other functions in
this module.

Ideally we want to switch modification to an atomic operations, so we
wouldn't need any locks in the getters.

intern/cycles/util/util_progress.h

@@ -226,6 +226,7 @@ public:
 
 	int get_current_sample()
 	{
+		thread_scoped_lock lock(progress_mutex);
 		/* Note that the value here always belongs to the last tile that updated,
 		 * so it's only useful if there is only one active tile. */
 		return current_tile_sample;
@@ -233,11 +234,13 @@ public:
 
 	int get_rendered_tiles()
 	{
+		thread_scoped_lock lock(progress_mutex);
 		return rendered_tiles;
 	}
 
 	int get_denoised_tiles()
 	{
+		thread_scoped_lock lock(progress_mutex);
 		return denoised_tiles;
 	}