Fix defect of export CgFx blend function.

After this fix. We can get blend function.


Ex. After
```
        <technique sid="SubtractiveBlend">
          <pass sid="p0">
            <depth_test_enable value="true"/>
            <depth_func value="LEQUAL"/>
            <blend_enable value="true"/>
            <blend_func><src value="SRC_ALPHA" /><dest value="ONE" /></blend_func>
            <blend_equation value="FUNC_REVERSE_SUBTRACT"/>
```

Ex. Before
```
        <technique sid="SubtractiveBlend">
          <pass sid="p0">
            <depth_test_enable value="true"/>
            <depth_func value="LEQUAL"/>
            <blend_enable value="true"/>
            <blend_func value="UNKNOWN INPUT ERROR" value="UNKNOWN INPUT ERROR"/>
            <blend_equation value="UNKNOWN INPUT ERROR"/>
```