fix shadervar return type error

about the issue #1312 

in the function, i don't think the `if (left == right)` is right. Or change the function like this:

```
protected ShaderVar getReturnTypeForOperation(DataType left, DataType right)
{
    ShaderVar out = null;

    if (left == right)
        out = getInstanceForDataType(left);
    else
        out = getInstanceForDataType(DataType.INT);

    return out;
}
```