When RPi initially released it's was was fully proprietary with only open source library that talked to the driver. Most of code and OpenGL ES implementation itself simply run outside of Linux on specialized core.
I suppose it's part of the same blob that manage both bootloader startup, display control, etc. RPi SoC built the way when GPU initialized before everything else and manage the boot process.
Then they released source for the actual OES implementationm but since it's run on core with limited capabilities it's hard to improve it much. Shortly after Broadcom hired Eric Anholt to work open source kernel driver and Mesa-based GL implementation that will actually run on Linux.
The problem is that their documentation contains only selected information and every time the community needs information about some other part they have to beg Broadcom for information. Most often the request was denied.
So here we are, with a partially functioning linux on an overheating pi3 that no one knows how to fix.
