Added code to draw current selection rectangle in viewport.

When the viewport is in selection mode, there is now some visual
feedback indicating what the selection will contain.

Change-Id: I118c47250dd3e56047227b1a4a483de0d30d96b0
Merge branch 'vtkweb-add-selection-rectangle-to-viewport-rel' into release

Change-Id: I8ab5c3a36f2e1649cee91552b1a5c3f1be277dec