Fix TestMultipleRenderers on Dash3's continuous

Apparently this test has been failing on dash3 since October.
This problem is ultimately due to the fact that we've enabled
the depth buffer within vtkOpenGLContextDevice3D::Begin().
This commit changes this behavior, so we now enable the depth
buffer at the beginning of each 3D context "Draw" method, and
then disable it at the end.

While investigating this problem, I realized that we copied
code from vtkOpenGLContextDevice2D and pasted it into the new
3D device class.  It turns out that this is unnecessary, so
this commit removes the copied & pasted code from
vtkOpenGLContextDevice3D.

Change-Id: If3ec870cedc4eb853a9a8cebba28663ef7137b3f
Merge topic 'fix_continuous_failing_test' into master

28b59275 Fix TestMultipleRenderers on Dash3's continuous