egl, mesa: more pbuffer fixes
Now that we've disabled the 565 pbuffers, the OpenGL CTS has started using the 8888 pbuffers. It turns out those are also broken.
- We were making both double-buffered and single-buffered pbuffer EGL configs. One of those things makes no sense.
- GetFramebufferAttachmentParameter needs to handle GL_BACK -> FRONT_LEFT translation in desktop GL (not just GLES).
- My earlier ReadBuffer fixes used DrawBuffer instead of ReadBuffer (found by inspection).