dont use glsl reserved keywords as variable names

[jciehl]

i've made a few modifications to make the radeon driver (fglrx 12.9) compiles brdftemplateAnglePlotAlbedo.vert, and measured.func, ie don't use half as a variable name and proper float litterals (0.0)

[Gregory Nichols]

I'm happy to incorporate those changes - can you create a pull request so I can bring them in?