Code refactor: zero render buffers outside of kernel.

This was originally done with the first sample in the kernel for better
performance, but it doesn't work anymore with atomics. Any benefit was
very minor anyway, too small to measure it seems.

intern/cycles/render/buffers.cpp

@@ -145,6 +145,13 @@ void RenderBuffers::reset(Device *device, BufferParams& params_)
 	device->mem_zero(buffer);
 }
 
+void RenderBuffers::zero(Device *device)
+{
+	if(buffer.device_pointer) {
+		device->mem_zero(buffer);
+	}
+}
+
 bool RenderBuffers::copy_from_device(Device *from_device)
 {
 	if(!buffer.device_pointer)