gl_FrontFacing — indicates whether a primitive is front or back facing
in bool gl_FrontFacing ;
Available only in the fragment language, gl_FrontFacing is an input variable
whose value is true if the fragment belongs to a front-facing primitive and
false otherwise. The determination of whether a triangle primitive is front-facing is made by
examining the sign of the area of the triangle, including a possible reversal of this sign
as controlled by glFrontFace. One way to compute this area is:
where