As jar mentioned, OpenGL is not a graphics driver. It's an industry standard API maintained by the Khronos Group. There are several software-only implementations available and the only requirement is that the rendering that appears on the frame buffer is compliant with the specification. The ARM cores (Cortex-A9) on the Zynq render to a dumb frame buffer on the Parallella and there is no hardware acceleration because there is no GPU on the Zynq (well, there may be some assist from the NEON SIMD instructions on the Cortex-A9, but that's a fine point). The cores on the Epiphany are not involved at all.
If you are truly interested in GPUs and graphics APIs (Vulkan, OpenGL, etc.) then a good project would be to develop a Gallium based driver making use of the Epiphany cores as best you can. As I mentioned in another post, a similar approach was taken with the Cell SPUs. Many of the math intensive graphics operations were offloaded to the SPUs leaving the PPU core to orchestrate. I believe the same general approach could work with Epiphany, though your results might vary. I suspect the much larger scratch memory on the SPUs, 128-bit registers and rich SIMD ISA, and higher bandwidth DMA engines would give the Cell SPU driver much improved performance over what you could obtain from Epiphany, but Cell is dead and you work with what you have.
You have your work cut out for you.