Shrimp Shader Generator FLTK2 GUI (SVN)

Created 'trunk' directory to make room for branches

Added abs() of color type (per component) to shrimp helpers header (together with color exp(), and single argument min() and max()).
Rewrote layering header, could be expressed in a more clean and faster way, and added new blending mode functions, operating on a per-component basis where needed. Added isolated blocks for each blending mode. Rewrote multiple choice block, so that the user set mode test is inside the block. Reference for blending modes is in 3 rsl_link tags, but currently only one is read for the help pages.

- add 2 separate filters in the file browser tif,tiff and tex,tdl,tx.
- fix path/name was passed to the shader without quotes.

FIX: TIFF preview now works properly

ENH: uncommented Freddy's work on TIF file preview, it works for me

-fix crash when select a texture file using the new file dialog in block attribute.

- Fix a bug in preference dialog the rib selection choice wasn't apply
- Add file dialog for choosing image (for example environment map)
- I had the preview of tif and tdl images in the file dialog but the code it's commented for memory issues.
- Add libtiff in Sconstruct for supporting tif and tdl preview but the code it's commented as well.

FIX: RIB scenes were appended to the list each time the preference dialog was opened

-update usage section with information of the blending mode number for each mode

- New block in a new section call Layering :blend color ,blend two color using the selected blending mode