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