Implement gl_FrontFacing for fragment shaders.

For the time being, we put the gl_FrontFacing value in the FOGC.Y input
register.  Combining FOGC and FrontFacing in one register is a bit of a
hack and may need to be changed someday.

src/mesa/swrast/s_fragprog.c

@@ -113,6 +113,10 @@ init_machine(GLcontext *ctx, struct gl_program_machine *machine,
 
    /* Setup pointer to input attributes */
    machine->Attribs = span->array->attribs;
+
+   /* Store front/back facing value in register FOGC.Y */
+   machine->Attribs[FRAG_ATTRIB_FOGC][col][1] = (GLfloat) ctx->_Facing;
+
    machine->CurElement = col;
 
    /* init condition codes */