Added Camera::isRenderToTextureCamera() method, and improved support in PickVisitor

and IntersectVisitor for CameraNode, including the ignoring of render to texture cameras,
such that HUD's etc are still intersected against.

src/osg/CameraNode.cpp

@@ -53,6 +53,11 @@ CameraNode::~CameraNode()
 {
 }
 
+bool CameraNode::isRenderToTextureCamera() const
+{
+    return (!_bufferAttachmentMap.empty());
+}
+
 void CameraNode::setRenderTargetImplementation(RenderTargetImplementation impl)
 {
     _renderTargetImplementation = impl;