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...
initializes the array to the number of graphics contexts, and automatically
expands the array when indices outside the current size are required.
Added new osg::Texture::Extensions nested class to handle extensions on a per
context basis.
