VideoCommon: Don't specify the redundant in/out qualifier if GL_ARB_shading_language_420pack is supported.

Some driver developers interpreted "can" as "must" in the OpenGL specs. (I'm looking at you AMD)
Merge pull request #1717 from Armada651/geom-shaderpack

VideoCommon: Don't specify the redundant in/out qualifier if GL_ARB_shading_language_420pack is supported.