Returns or sets the definition path as a String. The definition path is where the
shader was defined, so for parser-based definitions, this means that the file that was parsed
(eg., \Application\Shaders\cgfx\check3d.cgfx), whereas the plug-in based
definition paths refer to the plug-in file where the definition was implemented
(eg., \Application\Plugins\MyShader.dll), and the spdl-based definition paths
point to the SPDL files (eg., \Application\Shaders\spdl\sitoon\sitoon_paint.spdl).
