And now glReadPixels works on ATI pbuffer

glReadPixels from a pbuffer didn't work on a Radeon 9600 with the Linux fglrx drivers. And it was a very strange bug. Apparently pbuffer contexts don't work on ATI fglrx + R300 if you don't first create a X window context and make it current. Here's the code for the fix in nsGLPbufferGLX::Init:

// workaround for Radeon 9600 pbuffers contexts not working without a 
// previous window context
XVisualInfo *visinfo;
int vattrib[] = { GLX_RGBA, None };
visinfo = gGLXWrap.fChooseVisual( sharedDisplay, DefaultScreen(sharedDisplay), vattrib );
workaroundCtx = gGLXWrap.fCreateContext( sharedDisplay, visinfo, NULL, GL_TRUE );
gGLXWrap.fMakeContextCurrent( 
                        sharedDisplay, 
                        DefaultRootWindow(sharedDisplay), 
                        DefaultRootWindow(sharedDisplay), 
                        workaroundCtx );
XFree(visinfo);

Now my Canvas 3D demos work on ATI + Linux, yay!