vtkWeb: Guard against setting viewport size to zero

Setting viewport to zero in either dimension cause segfault in X.
Add guard to prevent setting size if zero values are provided.

Change-Id: Idf4c60ef82cd371c3d8d2c5dab8a30ea4917e3c9
Merge topic 'vtkweb-guard-viewport-size' into master

22e1caf8 vtkWeb: Guard against setting viewport size to zero