[Metisse] Best default depth

[Nicolas Roussel]

> In the last commit on nucleo/metisse, I've seen some changes in  
> documentation about default depth : do you think we should continue  
> to use 24bpp by default or use 16bpp instead ?

When the server runs in 24bpp mode, the pixels can now be sent  
directly to the compositor in ARGB format (before that, they were  
"translated" to RGBA). This means that the whole server+compositor  
might run faster in 24bpp now.

It would be interesting to measure the performance of a 24bpp  
configuration and compare it to a 16bpp configuration. x11perf might  
be a good start. Any other idea?

Nicolas