glsl: Add a new ir_unop_round_even opcode for GLSL 1.30's roundEven.

Also, update ir_to_mesa's "1.30 is unsupported" case to "handle" it.

src/glsl/ir.h

@@ -700,6 +700,7 @@ enum ir_expression_operation {
    ir_unop_ceil,
    ir_unop_floor,
    ir_unop_fract,
+   ir_unop_round_even,
    /*@}*/
 
    /**