Added aov_opacity AOV to root block source and AOV headers, and Thin Film Interference block (spec6), based on Jens's VEX shader. Tweaked rgb<>spec6 functions for Aqsis workarounds (component access can only be made with (xyz)comp in Aqsis). Both Aqsis and Pixie seem to accept arrays as function arguments, but there are some problems yet with rgb<>spec6 in both Aqsis and Pixie.
Glass block tweaked for Aqsis and Pixie workarounds (component access, array initialization, ternary operator expressions). Added missing vector exponent, maximum, and minimum functions to helpers header.
