Added support for using GL_UNPACK_ROW_LENGTH in conjunction with texture's + osg::Image via new RowLength

parameter in osg::Image.  To support this Image::setData(..) now has a new optional rowLength parameter which
defaults to 0, which provides the original behaviour, Image::setRowLength(int) and int Image::getRowLength() are also provided.

With the introduction of RowLength support in osg::Image it is now possible to create a sub image where
the t size of the image are smaller than the row length, useful for when you have a large image on the CPU
and which to use a small portion of it on the GPU.  However, when these sub images are created the data
within the image is no longer contiguous so data access can no longer assume that all the data is in
one block.  The new method Image::isDataContiguous() enables the user to check whether the data is contiguous,
and if not one can either access the data row by row using Image::data(column,row,image) accessor, or use the
new Image::DataIterator for stepping through each block on memory assocatied with the image.

To support the possibility of non contiguous osg::Image usage of image objects has had to be updated to
check DataContiguous and handle the case or use access via the DataIerator or by row by row.  To achieve
this a relatively large number of files has had to be modified, in particular the texture classes and
image plugins that doing writing.

src/osg/Texture2DArray.cpp

@@ -455,6 +455,7 @@ void Texture2DArray::applyTexImage2DArray_subload(State& state, Image* image, GL
     }    
     
     glPixelStorei(GL_UNPACK_ALIGNMENT,image->getPacking());
+    glPixelStorei(GL_UNPACK_ROW_LENGTH,image->getRowLength());
 
     bool useHardwareMipmapGeneration = 
         !image->isMipmap() && _useHardwareMipMapGeneration && texExtensions->isGenerateMipMapSupported();