FIX: fix serveral issues in multi-plate slicing

1. fix the thumbnail white issue in some random case
2. fix the thumbnail not update issue under preview panel
3. fix the previous panel's toolpath not clear issue while switch panel
   under preview
4. fix the toolpath not clear issue while switch from 3d to preview in
   some corner case

Change-Id: I158eba35e3b5d8a1e77f19ee9f86801e6a6dbd86

src/libslic3r/GCode/ThumbnailData.cpp

@@ -12,6 +12,7 @@ void ThumbnailData::set(unsigned int w, unsigned int h)
         width = w;
         height = h;
         // defaults to white texture
+        pixels.clear();
         pixels = std::vector<unsigned char>(width * height * 4, 255);
     }
 }