Fix detection of X.

ParaView was using obsolete PARAVIEW_USE_X flag. That flag was removed
during modularization changes. Fixed to use the proper VTK_USE_X flag to
check if we should use X API.

This now fixes the detection of a valid display on the server nodes.

Change-Id: I9478f991f81825fd3809895a311a0d3ce4e82043
Fix lost string termination character.

The code to send server-side messages to client was losing the string
termination character resulting in the client showing trailing garbage.
Fixed that.

Change-Id: Ibd6aec9fdcc488e2e7cd7c9564461580d457c645
BUG #13749: Auto-detect remote-rendering requirements.

Added support to vtkPVRenderView to enable representations to request
either remote or local rendering. vtkImageVolumeRepresentation uses this
new mechanism to tell the view that volume rendering requires remote
rendering. Now, when in client-server (or distributed) mode, a volume
rendering an image data, ParaView will automatically switch to remote
rendering, if possible. When not possible, we also display an error
message.

Also updated ImageVolumeRendering test to no longer manually force
remote-rendering.

Change-Id: I973b55f6bcaa1348c9e6f9c1739334dfc3318c47
Merge topic 'miscellaneous_volume_rendering_fixes' into next

c14d006f BUG #13749: Auto-detect remote-rendering requirements.
637e1399 Fix lost string termination character.
28ce06c0 Fix detection of X.