From Paul Martz, "We discussed the issue of GL_RED_SNORM and GL_RG_SNORM some time ago, but the issue was never completely resolved. Please see the attached submission to fix the issue.

osg/GL2Extensions was incorrectly defining GL_RED_SNORM and GL_RG_SNORM as part of the definitions for OpenGL v3.1. However, a quick review of the 3.1 spec indicates that these are not part of the 3.1 standard.

My attached change moves these definitions out of the #ifndef GL_VERSION_3_1 conditional block, and defines them conditionally if not already defined. This allows the DDS plugin to build for GL3.
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