Provide a method to set the sample spacing for slab modes.

The slab modes of vtkImageReslice allow for "thick slab" views, where
several slices are combined, for example, to create a maximum intensity
projection.  Previously the spacing between these slices was always set
to the output slice spacing, which was fine if the user only wanted to
create a single "thick slab", but less useful if the user wanted to create
a series of "thick slabs".  This patch adds a new method, called
SetSlabSpacingFraction, so that the user can first set the OutputSpacing
to set the spacing between the output slabs, and can then set the
SlabSpacingFraction to set the spacing between the slices that are
combined to make those slabs.

Change-Id: Ib70e92da9b1e27ba59a7a293d6e19b51ac25c9bb
mpi4py: link to CMAKE_DL_LIBS

Required on one of the dashboards at least.

Change-Id: I96d2ddb5351ef8d5c9d9f3ced52c74cac82d9d4d
Merge topic 'mpi4py-dlopen-link' into master

1493ff0c mpi4py: link to CMAKE_DL_LIBS
Replace an import which disappeared and broke all dashboard web tests.

Change-Id: I04ebb30c8900414ef98b8032d2cebe92dd669a6e
Merge topic 'pvweb-replace-missing-server-import' into master

2d1df149 Replace an import which disappeared and broke all dashboard web tests.
Save the rendering backend used to the config file

This detail is important for projects using VTK, it will also be used
to populate the VTK_LIBRARIES variable with implementations for any
selected modules.

Change-Id: I4f15f9bb221061ab369b5970200617b2dea66b50
Merge topic 'reslice-slab-fraction' into master

f1a720c9 Provide a method to set the sample spacing for slab modes.
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
Merge topic 'configure-backend' into master

0e8c8e14 Save the rendering backend used to the config file
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