Shrimp Shader Generator FLTK2 GUI (SVN)

Added some more fixes to blocks. Note the in 4D fractal noise, cellnoise, etc... typically the global variable time is used (which is the block's default as well), as provided by RiShutter, and storage class is uniform. There's some work ahead.

Added some more refinements to the blocks: adding more detailed usage information, or just adding it when missing, and fixing xml special chars. Tweaking funcionality a bit where required, and generally making things easier to read for the user.

Updated blocks, small fixes, workarounds for some renderers, small extensions for specific renderers, updated usage information as needed. Fixing XML special chars inside usage as well.

Small typo, forgot the & special character.

Modified the diffuse block to add the option to use Pixie's diffuse() in its 2nd form, with 3 arguments (surface point, normal, and angle).

Update to filterstep() and spline() shadeops, added extra filters supported by Pixie: blackman-harris, mitchell, sinc.

Added __nondiffuse and __nonspecular to light shaders, local illumination models proceed according to return status from lightsource() and these variable's content. Fixed cracked glazed clay example and point initialization in Voronoi blocks.

Added some fixes and tweaks to shading models headers, and extras.

Added the Buratti and Lommel-Seeliger blocks as well as their respective functions in the shading models header. Added some useful constants in helpers. Tweaked the shading models and distributions in their respective headers, into hopefully a more readable code. Added a note on the Cook-Torrance block regarding normalization of the Trowbridge-Reitz microfacet distribution.

FIX: unsupported display could be assigned when changing render engine from the main window