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: I8e144ed91c60caf216ad2dac0dce82d8e82ca14a
Merge topic 'vtkweb-add-selection-rectangle-to-viewport' into master

0d8d9933 Added code to draw current selection rectangle in viewport.