Compound variants don't look up final value of each variant if it's determined by media

[tkow]

I found that the variant prop value can be object in this line when medias are specified. So, even if each variants have been determined by media, it's not applied.

[tkow]

@Temzasse I can fix this, and finished working this PR. ~I'll do this if you want it.~ I've done that there.