[Xcb] profiling and performance

[Vincent Torri]

>
> It's interesting that you're calling both XCBPutImage and
> XCBShmPutImage. Is there a bug preventing you from always using shared
> memory?

I've looked at it. raster does not use shm if the part that is modified is
too small. It seems that it has a big impact on the performances (on my
computer, 30 fps !)

Vincent