[osg-submissions] Fixes in osgShadow::ShadowMap

[Sebastian Messerschmidt]

Hi Robert,

Are you going to merge these changes? You asked me to merge with the 
current SVN a couple of days ago so I replied with the new submission.
In the SVN however these changes are excluded. I know you still have 
your doubts about changes in this section due to the current troubles. 
But I believe that this patch is more an enhancement that doesn't touch 
the basic algorithm. Which is why I request you to take a look over it.

cheers
psy
> Hi,
>
> I've merged my changes with the current SVN and experienced some 
> strange things too.
> Wojciech I think has added negative polygonoffset when culling back 
> faces. This leads to selfshadowing everywhere on my ATI card.
> When supplying positive values however it works on ATI and NVIDIA 
> (when choosing a bigger value though)
>
> I've attached the merge as is, since my changes involve a function to 
> set the polyoffset from outside (see attached description for further 
> details).
> I also agree to Robert, that we need definitely more unit tests for 
> this, but I have that strange feeling, that won't be enough, since I 
> had very different experiences with different card/driver 
> combinations. (e.g. ATI .. they have inverted the t-coordinate of the 
> FBO-Texture in their newest drivers ...)
>
> I hope my changes are of any value.
>
> cheers
> psy
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