Cycles/BI: Add a pixel size option for speeding up viewport rendering

This patch adds "Pixel Size" to the performance options, which allows to render
in a smaller resolution, which is especially useful for displays with high DPI.

Reviewers: Severin, dingto, sergey, brecht

Reviewed By: brecht

Subscribers: Severin, venomgfx, eyecandy, brecht

Differential Revision: https://developer.blender.org/D1619

intern/cycles/render/session.cpp

@@ -46,7 +46,7 @@ Session::Session(const SessionParams& params_)
 : params(params_),
   tile_manager(params.progressive, params.samples, params.tile_size, params.start_resolution,
        params.background == false || params.progressive_refine, params.background, params.tile_order,
-       max(params.device.multi_devices.size(), 1)),
+       max(params.device.multi_devices.size(), 1), params.pixel_size),
   stats()
 {
 	device_use_gl = ((params.device.type != DEVICE_CPU) && !params.background);