sampler2Ds must be uniform

This is valid GLSL:

```glsl
uniform sampler2D samp;
vec3 dof(sampler2D tex) {
    return texture(tex,vec2(0)).rgb;
}

// ...
dof(samp);
```

This is not valid:

```glsl
uniform sampler2D samp;
vec3 dof() {
    sampler2D tex = samp;
    return texture(tex,vec2(0)).rgb;
}
```

When inlining function parameters, Shader Minifier might introduce a `sampler2D` variable which is not allowed.
