2) Changed Makedepend system to make individual dependency files, which
should save time rebuilding dependencies by limiting the regeneration
of dependency file for only modified source files.
osg::Image::setImage has also been modified to require the AllocationMode
mode to be passed to it so that it knows how to delete the data once the
image goes out of scope.
Port the image plugins across to specify the new AllocationMode, and converted
them across to using new/delete in place of malloc/free.
