_{Note: This community wiki answer can be deleted if chrisl posts an answer}

Quoting chrisl's comment which seems to be the answer:

I found this file at github by searching for the name of the header file in addition to "github". renderBitmap is a define, which used loadPixels() (line 145).

You didn't find it in Documents/Arduino/libraries, because that folder is for the libraries, that you installed. But that header file belongs to the renesas core, so you will find it in the core packages