[Xcb] profiling and performance

[Vincent Torri]

> Hey, it just occurred to me: would you retest Xlib with the environment
> variable XLIBBUFFERSIZE set to 4? If its performance drops to match
> XCB's then we'll know it's XCB's smaller output buffer that's the cause
> of the performance difference.

Indeed, the fps of the Xlib engine has dropped a lot. The xcb engine has
just a fps which is very close to the Xlib fps, now. A bit below, but not
much.

i'll try to get better fps, nevertheless, with your other advices

Vincent