Add a little USE_DIA for anyone building on VS express to disable

* The DIA SDK is only available on VS pro and above, but it's only needed
  to be able to resolve symbols in the pdblocate helper program. So we add
  a little define that makes pdblocate instantly quit, and then symbol
  resolution just fails.
Update README in a few places

* Tweaked links and rearranged a few things
* Updated the building section with instructions for VS2013 or express
Experimental PixelHistorTree
dialog
Sort-of-kind-of working histortree UI, launched from texture viewer
Changes for more complex demos
Be a tad more forgiving about finding equivalent pixels
A few more tweaks and optimisations
Much as I like puns, rename from PixelHistorTree to PixelShadeGraph