Overhaul SearchApiFieldTrait::preRender()

There are already several issues for this, with one common theme: while our SearchApiFieldTrait::preRender() basically works (as amazing as that is), it's unoptimized, has some problems in edge cases and looks horrible. (I get the shivers every time I'm forced to touch it – or, worse, to debug it.)

If someone could at some point write a better replacement for it, even if it just fixes one of the three problems, that would be great.