Bug in variable minification : field property get accidentally minified

With this glsl:

``` glsl
void foo (vec2 p) {
  float x = mix(p.x, 1.0-p.x, 0.4);
}
```

Here is the result:

```
$ cat bug.glsl | bin/glslmin
void a(vec2 b){float c=mix(b.c,1.0-b.c,0.4);}
```

Which is wrong. It should be:

```
void a(vec2 b){float c=mix(b.x,1.0-b.x,0.4);}
```
