Use back buffer and vtkWindowToImageFilter for GL2PS rasterization.

This fixes some regression test failures that were showing overlapped
windows in the rasterized bits of the postscript.

Change-Id: Ib1da40e92334f6a19e7ad909398f55f4511a1345
Merge topic 'gl2ps-dashboard-fixes' into master

e3fb0886 Use back buffer and vtkWindowToImageFilter for GL2PS rasterization.