From Wojciech Lewandowski, "I did few tests to see whether adding PreDraw callback would help us with SPI problems when using Viewer. Results were positive so I decided to give it a try and ask you to verify and maybe merge with existing codebase.

I added _preDrawCallback member and neccessary access methods plus modified osgUtil RenderStage.cpp to invoke it before all drawInner calls are made. I tried to maintain symmetry with postDrawCallback but you know better where is a proper place for this call ;-)
"

src/osg/Camera.cpp

@@ -52,6 +52,7 @@ Camera::Camera(const Camera& camera,const CopyOp& copyop):
     _renderTargetImplementation(camera._renderTargetImplementation),
     _renderTargetFallback(camera._renderTargetFallback),
     _bufferAttachmentMap(camera._bufferAttachmentMap),
+    _preDrawCallback(camera._preDrawCallback),
     _postDrawCallback(camera._postDrawCallback)
 {
     // need to copy/share graphics context?