Fix memory leak in` vtkWebApplication::StillRender`

Christos Tsolakisrequested to merge
christos.tsolakis/vtk:release-fix-vtkwebapplication-memory-leak into master

vtkWebApplication::StillRender was passing a raw vtkImageData* to vtkDataEncoder::Push causing a leak. A test is added that reports memory leaks when compiled with VTK_DEBUG_LEAKS on current master but is fixed in this MR.

Backport: release

Edited by Christos Tsolakis

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