Improved handling of subloading.

Removed the rescalling of images in osg::Image during texture apply, moving
the rescale so it is locally calculated.  This solves an outstanding threading
problem which occured by multiple draw threads all tried to rescale the same
image at one time.

Made osg::Image ptr in osg::Texture2D non mutable as it is no longer modified
during apply.

include/osg/Texture2D

@@ -123,7 +123,7 @@ class SG_EXPORT Texture2D : public Texture
         // Image::ensureDimensionsArePowerOfTwo() can only be called
         // in a valid OpenGL context, a therefore within an Texture::apply
         // which is const...
-        mutable ref_ptr<Image> _image;
+        ref_ptr<Image> _image;
 
         // subloaded images can have different texture and image sizes.
         mutable GLsizei _textureWidth, _textureHeight;