The SoC vendors are willing to target only Android
Android GPU drivers are built against Bionic libc
The GPU drivers talk to hardware, and expose themselves via EGL and GLESv2
EGL is basically a common API for GPU memory management, buffer (region of memory used for rendering) allocation and display updates
GLESv2 stands in for the functionality we commonly associate with OpenGL
GPU drivers form a combination of EGL and GLESv2 libraries, each GPU vendor providing their own
This is where libhybris comes into play. The GPU driver libraries don't work without Bionic libc - so libhybris, while running on top of regular linux (and thus [e]glibc), keeps a private Bionic libc open for the GPU drivers' use, and redirects all the EGL/GLESv2 calls to the GPU driver libraries. These libraries run in their own Bionic universe, and tell the actual display hardware what to do.