Added support for specifying split mode.

When the vtkStreamingDemandDrivenPipeline automatically
partitions a structured request because of CAN_PRODUCE_SUB_EXTENT
being set, it always used vtkExtentTranslator::BLOCK_MODE. I added
support for downstream filters to specify a split mode to be used
as an alternative. This is done by setting the UPDATE_SPLIT_MODE
key in RequestUpdateExtent.

Change-Id: Ie8dc2af898352adeaa261405d483ac619c035021
Merge topic 'split-mode-request' into master

d8bdbb60 Added support for specifying split mode.