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

release/scripts/startup/bl_ui/properties_render.py

@@ -275,6 +275,7 @@ class RENDER_PT_performance(RenderButtonsPanel, Panel):
 
         col.separator()
         col.prop(rd, "preview_start_resolution")
+        col.prop(rd, "preview_pixel_size", text="")
 
         col = split.column()
         col.label(text="Memory:")