1) Add getShadowComparison() accessor function to osg::Texture class
2) Modify ReaderWriterTiff::writeTifStream() and _readColor() (in Image.cpp) to handle pixelFormat==GL_DEPTH_COMPONENT as if it were GL_LUMINANCE
3) Modify the Texture classes of the ive and osg plug-ins so that they save/load the following Texture members: _use_shadow_comparison, _shadow_compare_func and _shadow_texture_mode
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