Fixed the texture apply method which was broken yesterday, the new code wasn't

checking for the validity of _image before doing checks on it.  This breaks when
render to texture is used as there is no image to check against...

Added osgshadowtexture demo source.  Demos still in development, so don't expect
any shadows yet...

src/osg/Texture2D.cpp

@@ -94,7 +94,7 @@ void Texture2D::apply(State& state) const
         {
             _subloadCallback->subload(*this,state);
         }
-        else if(getModifiedTag(contextID) != _image->getModifiedTag())
+        else if (_image.valid() && getModifiedTag(contextID) != _image->getModifiedTag())
         {
             applyTexImage2D(GL_TEXTURE_2D,_image.get(),state,
                             _textureWidth, _textureHeight, _numMimpmapLevels);