Cycles: Refactor Progress system to provide better estimates

The Progress system in Cycles had two limitations so far:
 - It just counted tiles, but ignored their size. For example, when rendering a 600x500 image with 512x512 tiles, the right 88x500 tile would count for 50% of the progress, although it only covers 15% of the image.
 - Scene update time was incorrectly counted as rendering time - therefore, the remaining time started very long and gradually decreased.

This patch fixes both problems:
First of all, the Progress now has a function to ignore time spans, and that is used to ignore scene update time.
The larger change is the tile size: Instead of counting samples per tile, so that the final value is num_samples*num_tiles, the code now counts every sample for every pixel, so that the final value is num_samples*num_pixels.

Along with that, some unused variables were removed from the Progress and Session classes.

Reviewers: brecht, sergey, #cycles

Subscribers: brecht, candreacchio, sergey

Differential Revision: https://developer.blender.org/D2214

intern/cycles/device/device_multi.cpp

@@ -89,6 +89,14 @@ public:
 		return error_msg;
 	}
 
+	virtual bool show_samples() const
+	{
+		if(devices.size() > 1) {
+			return false;
+		}
+		return devices.front().device->show_samples();
+	}
+
 	bool load_kernels(const DeviceRequestedFeatures& requested_features)
 	{
 		foreach(SubDevice& sub, devices)