Cycles: multi GPU rendering support.

The rendering device is now set in User Preferences > System, where you can
choose between OpenCL/CUDA and devices. Per scene you can then still choose
to use CPU or GPU rendering.

Load balancing still needs to be improved, now it just splits the entire
render in two, that will be done in a separate commit.

intern/cycles/render/buffers.cpp

@@ -87,7 +87,7 @@ float4 *RenderBuffers::copy_from_device(float exposure, int sample)
 	if(!buffer.device_pointer)
 		return NULL;
 
-	device->mem_copy_from(buffer, 0, buffer.memory_size());
+	device->mem_copy_from(buffer, 0, params.width, params.height, sizeof(float4));
 
 	float4 *out = new float4[params.width*params.height];
 	float4 *in = (float4*)buffer.data_pointer;