add depth peeling

Change-Id: Ic3f725b5c7ef2ed7f7d639acaba5a727bb1768b8
make wider so we can see errors

Change-Id: I22e79f4016eb95387ceeb1389faa276d099c00e1
Merge branch 'opengl2-rendering' of https://github.com/cryos/VTK into opengl2-rendering

Change-Id: I1d94353b7ecde0d7800edb797bc997e6f475808f