Added support for setting the texture subload width and height within

osg::Texture.  The values default to 0, and in this state they are
sizes of the glTexSubImage2D are taken from the source image size.  This
allows the previous setup with no settings for subload size to work as
before.

src/osg/TextureCubeMap.cpp

@@ -205,7 +205,7 @@ void TextureCubeMap::apply(State& state) const
                 {
                     glTexSubImage2D(faceTarget[n], 0,
                                     _subloadOffsX, _subloadOffsY,
-                                    _images[n]->s(), _images[n]->t(),
+                                    (_subloadWidth>0)?_subloadWidth:_images[n]->s(), (_subloadHeight>0)?_subloadHeight:_images[n]->t(),
                                     (GLenum) _images[n]->pixelFormat(), (GLenum) _images[n]->dataType(),
                                     _images[n]->data());
                     // update the modified flag to show that the image has been loaded.