initializes the array to the number of graphics contexts, and automatically
expands the array when indices outside the current size are required.
Added new osg::Texture::Extensions nested class to handle extensions on a per
context basis.
added osgtexture1D demo which currently is simply copy of osgtexture2D. I
will be modifying it to do 1D texturing next.
Fixed a bug in osg::Texture3D relating to checking of existance of
texturing.
Merged some fixes to the pfb loader from Ulrich Hertlein.
