[osg-submissions] SharedStateManager updated for all DataVariance modes

[Terry Welsh]

It sounded like nobody was opposed to this update, so here is the
code.  I didn't change the value of SHARE_ALL for the sake of backward
compatibility even though it probably should be different.  I still
consider that possibility open for discussion.

Per Robert's suggestion, the default share mode was SHARE_TEXTURES and
is now SHARE_STATIC_TEXTURES | SHARE_UNSPECIFIED_TEXTURES.  I'm not
sure why textures were the default share mode instead of StateSets.
Maybe the original author will comment on that?  I kept SHARE_TEXTURES
equal to SHARE_STATIC_TEXTURES for backward compatibility, and did the
same for StateSets.

After doing this I think SharedStateManager could be expanded a lot,
but I don't know how much it would help performance.  For example, you
could share individual StateAttributes instead of whole StateSets.
More specifically, a StateSet is not equal to another StateSet if only
its texture is different.  Seems like there would be a lot of
opportunities in typical OSG usage to share most attributes in
StateSets that only differ by their texture.
- Terry
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