Pull in Spire via submodule?

[tangent-vector]

The codebase currently has Spire.h and Spire.cpp copy-pasted in somewhat deep in the source tree. Updating to a new version of the main Spire repository is not automated.

I expect that a lot of developers would want to see an example of a simple workflow using git submodule, where the entire Spire tree is pulled into a simple location like External/Spire or ThirdParty/Spire, and can be updated to a new version (or switched to a different branch) painlessly.

Such a change would also make it easier to work on the Spire implementation and the engine at the same time, by removing a manual copying step.

[csyonghe]

The latest check-in is now using Spire as sub-directory (but not git submodule). Probably need to move to a upper directory for ease integration with submodules.