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A search will not find much yet on this.
Some are further along than others.
With the Pi4 OpenGL will be the starting point.
The source for that open driver is in the mesa code.
Not sure if or when VC6 OpenGLES/VG stuff will show up.
I already run OpenGL games, some are ok, some need work and some just don't run yet.
Grab the source for working ones and figure them out?
Start with the mesa demo examples, those are mostly single C files and easy to read.
I did get them working in Gentoo64 and 3B+, I have yet to get them working on a Pi4.
Also look on the baremetal posts, clues are poping up there too.
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges