Fix issues in Cycles standalone, particularly related to render display

Some code was removed to avoid storing the combined pass when viewport
rendering other passes. But we can keep this by default, Blender overrides
the list of passes entirely.

intern/cycles/render/buffers.cpp

@@ -43,6 +43,8 @@ BufferParams::BufferParams()
   denoising_data_pass = false;
   denoising_clean_pass = false;
   denoising_prefiltered_pass = false;
+
+  Pass::add(PASS_COMBINED, passes);
 }
 
 void BufferParams::get_offset_stride(int &offset, int &stride)