add inverseTransformNormal function to common.glsl and unfix bug with envmap_fragment.glsl
missing brace.
fix bug identified by @WestLangley
Address @zz85's concern about using the constant 0.33333 insteaed of a division: https://github.com/mrdoob/three.js/pull/5785#discussion_r22146657
common.glsl: transformNormal -> transformDirection and inverseTransformNormal -> inverseTransformDirection.
adopt attenuation equation discussed here: https://github.com/mrdoob/three.js/pull/5787#issuecomment-68066541  Also apply attenuation formula to example shaders.