Added new osg::DrawPixels class with encapsulates glDrawPixels as and

osg::Drawable.  Added osg::Image::readPixels to osg::Image.

Made osg::LightSource to default to cullActive set to false to that LightSource
nodes don't get culled by default.

src/osg/LightSource.cpp

@@ -4,7 +4,8 @@ using namespace osg;
 
 LightSource::LightSource()
 {
-    _bsphere_computed = false;
+    // switch off culling of light source nodes by default.
+    setCullingActive(false);
 }