As far as I know, no-one has yet shipped a product that has a graphics sw stack written with a GPGPU api like OpenCL. It's an interesting project to do and you

As far as I know, no-one has yet shipped a product that has a graphics sw stack written with a GPGPU api like OpenCL. It's an interesting project to do and you can do really cool stuff. However, practical it is not. You also shouldn't forget that the GPU has dedicated hardware for graphics that cannot be accessed through a GPGPU API. The framebuffer can't be manipulated with a GPGPU API. Window compositing is largely done by dedicated hardware. Some GPU's still have dedicated hardware for vertex shading. If you try to write a graphics stack with a GPGPU API, all this hardware is left unused.