glm::mix does not comply with GLSL specification

The GLSL specification of `mix(x, y, a)` says
```
Returns the linear blend of x and y, i.e., x⋅(1−a) + y⋅a.
```

However, the current glm implementation uses
```
x + a⋅(y-x)
```
, which does not guarantee `mix(x, y, 1) == y`.