vtkWeb: Don't reset viewer to null on updateViewer(...)

Only set m_viewer when updateViewer(...) return a new viewer. This
is necessary so that the m_viewer can be used for propagation
mouse interaction events.

Change-Id: I2d6186b97fbd8d44a43b83b2dc9bf93304fc50ad
vtkWeb: Propagate mouse events to VGL

Rather than replacing the document event handlers bind a new event
handler than will propagate the mouse events to the VGL viewer.

Change-Id: If6a10243dab167d03072d3dad31543435b0a9986
vtkWeb: Fix calls to old VGL API

Looks like this code was not being run. Fix up typos and API calls.

Change-Id: I6eafda86ae3cbb2361592a394f5d17816dc62aa7
Merge topic 'vgl-interaction-fixes' into master

175d3ddd vtkWeb: Fix calls to old VGL API
7b0dae91 vtkWeb: Propagate mouse events to VGL
59681720 vtkWeb: Don't reset viewer to null on updateViewer(...)