Added a new Gaussian blur function.

gdImageCopyGaussianBlurred() returns a blurred copy of its argument.
Blurring is done in two passes to take advantage of the Gaussian
function's separability for.  This makes the algorithm much faster
than single-pass implementations, especially for larger radii.

This change also adds documentation for the new function and the
existing blur (gdImageGaussianBlur()).
Added a test case for gdImageCopyGaussianBlurred().

This change is relatively thorough.  It confirms that the blurring
process succeeds and that the result is at least somewhat blurry.
Merge branch 'master' of https://bitbucket.org/libgd/gd-libgd