mesa: don't advertise bogus GL_ARB_shading_language_120 extension
Instead of using the invalid GL_ARB_shading_language_120 extension to determine the GLSL version, use a new ctx->Const.GLSLVersion field. Updated the intel and r600 drivers, but untested. See fd.o bug 29910 NOTE: This is a candidate for the 7.9 branch (but let's wait and see if there's any regressions).
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