make rubber band look correct on scenes with textures

vtkTexture turns on an alpha test. If your scene has any textures in
it the selection rubber band looks wrong. The reason is that when
rubber band draws into the screen with renderwindow::SetRGBA the background
pixels, presumably with failing alpha values, can not change.

Change-Id: I6c78592234b9b4e38e020f148def9d84d7d66bf8
Merge topic 'fix-rubber-band' into master

252798f0 make rubber band look correct on scenes with textures