[Xorg] DRI fixes for libdl module loader

[Adam Jackson]

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I've got DRI and GLX working with the libdl module loader under current Xorg 
CVS.  It works well enough to run glxgears (both direct and indirect) and 
quake3.  The patch isn't quite finished, for two reasons.  One, and rather 
trivially, since the GLX code is shared with X11.app and Cygwin/X, their 
indirect renderers need to be updated to match GLcore.  This is effectively a 
one-liner.

The second problem is trickier.  GLX is in principle independent of DRI; you 
can load it by itself and get just an indirect renderer in the server.  The 
reverse is not true, since atm DRI's functionality is implicitly tied to GL 
contexts in the server.  Clearly then, libglx has to load before libdri can 
do anything useful.

There are two ways of expressing this.  libglx could load libdri as a 
submodule, the way it does libGLcore; or vice versa, libdri loads libglx.  
With the first method, we'd need a way to suppress loading libdri (no 
applicable hardware, or just the user doesn't want it).  The 'omit' verb in 
the config file syntax is probably sufficient.  With the second method, 
libdri becomes the top of the stack.  I'm slightly in favor of the second 
method, one because it doesn't change the behaviour of existing config files, 
and two because it reinforces that DRI can be more than just a GL driver.  
(Not that anyone has taken that route.)

One of my goals with the libdl loader work is ensuring that the user can load 
any given module and it will work (if perhaps by doing nothing).  Without one 
of the two proposed changes above, configs that say Load "dri" before Load 
"glx" will fail to start, and that's not acceptable.

The relevant bug is http://freedesktop.org/bugzilla/show_bug.cgi?id=377 .  
I'll post patches there later tonight.

- - ajax
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