Add new procedural shader: CreateProceduralGaussBlob()

Creates procedurally shaded gaussian blobs, ie. the exp()onential
gauss bell curve shape. This is the same as our gabor shading,
except there isn't a sine wave modulated by an exponential hull,
but there is a "constant one" modulated by an exponential hull
- It's the hull without the sine grating.

Nice for drawing blobs with dynamic contrast, stddev, rotation
and aspect ratio.
ProceduralGarboriumDemo: Also demo the new "gaussian blobs"

Too lazy to write a separate demo, and this makes for more
variety in our little aquarium of patches.