Add a comprehensive library of Gaussian blur functions (and more)...

Add a comprehensive library of Gaussian blur functions, along with .cg
shaders that use them and testing/benchmarking .cgp presets.  Also add
srgb-helpers, a collection of canned .cg files for linearizing the first
pass and gamma-correcting the last pass differently depending on whether
another display is being simulated (e.g. a Game Boy Advance screen).  Both
rely on tex2D* wrappers in gamma-management.h to make it trivial to switch
between gamma-correct sRGB and non-sRGB pipelines.  Reusable headers are
kept in the include directory (currently blur functions, gamma management
wrappers, and pixel quad communication functions.)

A few more notes:
Some of these blurs (like the blurNfast family) are faster than the ones that are typically used, whereas other blurs have different advantages/tradeoffs.  All rely on static weighting for speed.

Many of the contents of this commit are tailored more toward shader authors than end users, but the wide variety of .cg files is meant to help knowledgeable end users add the right Gaussian blur to their ad-hoc shader chain too.  (More of these should become directly useful once users can set sRGB and mipmap settings from RGUI or similar too.)