Avoid early termination when collecting actors

This method returns NULL when there are no more objects, but it was
also resulting in NULL if the safe down cast failed. This could lead
to early termination when there is a mixture of different actors and
the context is not the first.

Change-Id: Ia1a13e0be1c89d41fb767c0c99aa064d977f9231
vtkOpenGLContextActor should subclass vtkContextActor

This was true for the OpenGL2 variant, but not OpenGL. This caused
the IO/Export tests to fail as they could not safe down cast the
actor in the prop collection. Also removed unneeded includes etc
from the parent class.

Change-Id: I1acd77e84df3d4a1362223bbc1581b626731c3da
One of the tests needs a context device to render

Change-Id: I3dad8993e7057782897ddbe1a232e2df376a91b5
Removed unused variable - used in subclasses

Change-Id: I312d6a1821dd5d24f402481c2d954fd200b64f19
These classes are abstract, fix smoke tests

Change-Id: I68abd385861aff46b230e14f5d498c30b3a4f79e
Fix for Windows mangling issue

Change-Id: Idc054cb8afbbb0eaae106ba575d3f5ee9670e03a
Merge topic 'test-fixes' into master

c3ffed03 Fix for Windows mangling issue
7ded8b1a These classes are abstract, fix smoke tests
94e1f9e0 Removed unused variable - used in subclasses
92b24859 One of the tests needs a context device to render
8b695d83 vtkOpenGLContextActor should subclass vtkContextActor
fdddd749 Avoid early termination when collecting actors