Add PixelValue implementations for Luma/LumaA
Add PixelValue implementations for Luma/LumaA

binsoftware/glium,msiglreith/glium,sdotz/glium,cpjreynolds/glium,carbidegames/glium,tomaka/glium,nicokoch/glium,glium/glium,christopheraugustus/glium,acolley/glium,bvssvni/glium,caldwell/glium,TyOverby/glium,rozaliev/glium