Bug 4689

When launching ppracer, the server gets a SEGV. I've tracked that down a bit. We
end up crashing in CreatePicture() called by compWindowUpdateAutomatic() (called
from compWindowUpdate() called from the BlockHandler). It's a nice NULL
dereference. The bug seem to be that CreatePicture() gets called with a NULL
pFormat pointer:

compWindowUpdateAutomatic (WindowPtr pWin)
{
    CompWindowPtr   cw = GetCompWindow (pWin);
    ScreenPtr       pScreen = pWin->drawable.pScreen;
    WindowPtr       pParent = pWin->parent;
    PixmapPtr       pSrcPixmap = (*pScreen->GetWindowPixmap) (pWin);
    PictFormatPtr   pSrcFormat = compWindowFormat (pWin);

*** Here, pSrcFormat is NULL ***

    PictFormatPtr   pDstFormat = compWindowFormat (pWin->parent);
    int             error;
    RegionPtr       pRegion = DamageRegion (cw->damage);
    PicturePtr      pSrcPicture = CreatePicture (0, &pSrcPixmap->drawable,
                                                 pSrcFormat, 
                                                 0, 0,
                                                 serverClient,
                                                 &error);
    
And the above CreatePicture crashes. Now why is it NULL ? Well, I've traced a
bit. It all ends up in PictureMatchVisual() which returns basically the pointer
we are getting.

First a note: There seem to be _plenty_ of cases where this can return NULL.
However, compWindowUpdateAutomatic() tests none of them. Shouldn't this function
be hardened a little bit ?

Now, why is it returning NULL ? Here is what it's called with:

pVisual points to visual ID 47 which is a visual of class 5 (DirectColor), 8
bits per RGB values, 256 colormap entries, 24 planes, looks like a 24 bits RGB
visual.

But ... Heh ! PictureMatchVisual() will always return 0 for a DirectColor visual
! It's in the switch/case...

So here, I don't know what _should_ happen as I don't completely understand the
purpose of the function here. What I can tell however, is that we are trying to
get the format pointer for a window with a DirectColor visual, which fails,
returns NULL, and passing that to CreatePicture.

So there are 2 things here:

 - compWindowUpdateAutomatic() should be hardened to bail out instead of
crashing when it gets a NULL result from compWindowFormat()

 - should compWindowFormat() (and thus maybe PictureMatchVisual) be fixed to
deal with DirectColor visuals ?