[osg-submissions] Fix for very slow mipmap generation

[Michael Platings]

Hi Robert,
On nvidia cards mipmap generation for non-power-of-two textures with
GL_GENERATE_MIPMAP_SGIS is very slow (over half a second for a 720*576
texture). However, glGenerateMipmapEXT() performs well (16ms for the
same texture), so I have modified the attached files to use
Texture::generateMipmap() if glGenerateMipmapEXT is supported, instead
of enabling & disabling GL_GENERATE_MIPMAP_SGIS.
The files have been modified from the 2.4 release
Thanks
-Michael Platings

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