Fixes #779 - merging `background-(position|size)`.

Fixes #779 - merging `background-(position|size)`.

Those two properties were always merged but if vendor-prefixed
functions are used then they should not be.

Note we changed `everyCombination` checker to check for values
on same position when merging **longhand** properties which is what
browsers do, e.g.

```css
background-position: calc(100% - 1em) 1em;
background-position: calc(100% - 2em) 2em;
```

Now `everyCombination` checks if 1st and 2nd values can be merged
separately and ignores whether 1st can be merged with 2nd.