PERF: Do not spawn threads that will not be used.

Update the ImageSource base class, to not spawn threads that will not
be used. Previously, threads were spawned and then terminated in the
ThreaderCallback method. This method was not user overindulge, so no
compatibility issues should be created.

I have observed up to 100X performance improvement for processing an
image of 1 pixel :)

Change-Id: Ie2d21886e175b5fbee0ce20f0628c97020d8d9e2
ENH: ImageIO's now use ImageRegionSplitter base

Moved ImageIORegion to Common module. Added GetModifiable methods.

Added methods to ImageRegionSplitterBase for ImageIORegion objects.

Refactored ImageIOBase to use the default ImageRegionSplitterBase
method for streaming IO.

Change-Id: I0c07db23824d65086a64b76dbecbe01137fa78a2
STYLE: Add more detail to the vxl update instructions

State explicitly that the upstream extraction script should be run in an
outside tree and the results moved into the temporary directory for
commit of a new upstream snapshot.

Change-Id: I1a9742a9154c8d45bb7bb530cd16e42687268a43
Merge topic 'ConsolidateSplitting_PerfThread'

0e9112f PERF: Do not spawn threads that will not be used.
Merge topic 'ConsolidateSplitting_IO'

defb9c1 ENH: ImageIO's now use ImageRegionSplitter base
COMP: Specify export for Specialized Array output operators

Change-Id: I436ec75abe86ad6f2956b9b883ee7c548e9c017e
Merge topic 'SpecializedTemplateVisibility'

ab363d0 COMP: Specify export for Specialized Array output operators
Merge topic 'vxl-update-instructions'

e245f0b STYLE: Add more detail to the vxl update instructions